,common ,minerals and ,rocks ---------------------------------------- -------------------------------- ,boston ,society of ,natural ,history ,,guides ,,for ,,science-,,teaching ,,no4 ,,xii ,,common ,,minerals ,,and ,,rocks ,,by ,,william ,o4 ,,crosby ,d4 ,c4 ,,heath ,,co41 ,,publishers ,,boston ,,new ,,york ,,chicago ---------------------------------------- -------------------------------- ,copyright ,,by ,,the ,,boston ,,society ,,of ,,natural ,,history #ahha ,i ,d #b ---------------------------------------- -------------------------------- ,,introduction4 ,illustration ,minerals and rocks1 or the inorganic portions of the earth1 constitute the proper field or subject-matter of the science of ,geology4 ,now the inorganic earth1 like an animal or plant1 may be and is studied in three quite distinct ways1 giving rise to three great divisions of geology1 which1 as will be seen1 correspond closely to the main divisions of ,biology4 ,first1 we may study the forces now operating upon and in the earth--the geological agencies--such as the ocean and atmosphere1 rivers1 rain and frosts1 earthquakes1 volcanoes1 hot springs1 etc41 and observe the various effects which they produce4 ,we are concerned here with the dynamics of the earth2 and this is the great division of dynamical geology1 corresponding to physiology among the biological sciences4 ,or1 second1 instead of geological causes1 we may study more particularly geological effects1 observing the different kinds of rocks and of rock-structure produced by the geological agencies1 not only at the present time1 but also during past ages4 ,this method of study gives us the important division of structural geology1 corresponding to anatomy and morphology4 ,all phenomena present two distinct and opposite aspects or phases which we call cause and effect2 and so in dynamical and structural geology we are really studying the opposite sides of essentially the same classes of phenomena4 ,in the first division we study the causes now in operation and observe their effects2 and then1 guided by the light of the experience thus gained1 we turn to the effects produced in the past and seek to refer them to their causes4 ,these two divisions together constitute what is properly known as physiography2 and they are both subordinate to the third great division of geology1--historical geology1--which corresponds to embryology4 ,the great object of the geologist is1 by studying the geological formations in regular order1 from the oldest up to the newest1 to work out1 in their proper sequence1 the events which constitute the earth's history2 and dynamical and structural geology are merely introductory chapters1 the alphabet1 as it were1 which must be learned before we are prepared to read understandingly the grand story of the geological record4 ,our work in this short course will be limited to the first two divisions1--i4e41 to dynamical and structural geology4 ,we will attempt1 first1 a general sketch of the forces now concerned in the formation of rocks and rock-structures2 and after that we will study the composition and other characteristics of the common minerals and rocks4 ,the scope of this work1 and its relations to the whole field of geology1 are more clearly indicated by the following classification of the geological sciences3-- ,,dynamical ,,geology ,physical ,geology4 ,chemical ,geology4 ,,geology ,,structural ,,geology ,mineralogy4 ,petrography ,lithology4 ,petrology4 ,,historical ,,geology4 ,many teachers will desire to fill in some of the details of the outline sketch presented in this ,guide1 and for this purpose the following works are especially recommended3-- ,,elements ,,of ,,geology4 ,by ,prof4 ,joseph ,le ,conte4 #ahhb4 ,d4 ,appleton ,co41 ,new ,york4 ,nearly #fjj pages4 ,,manual ,,of ,,geology4 ,by ,prof4 ,j4 ,d4 ,dana4 ,third edition4 #ahhj4 #hjj pages4 ,,text-,,book ,,of ,,geology4 ,by ,prof4 ,a4 ,geikie4 #ahhb4 ,macmillan ,co41 ,london4 ,nearly #ajjj pages4 ,as a reference-book for mineralogy1 the following treatise is unsurpassed3-- ,,text-,,book ,,of ,,mineralogy4 ,by ,edward ,s4 ,dana4 #ahhc4 ,john ,wiley ,sons1 ,new ,york4 ,and1 as an introduction to the study of minerals1 and1 through these1 to the study of rocks1-- ,,first ,,lessons ,,in ,,minerals4 ,science ,guide ,no4 ,,xiii4 ,by ,mrs4 ,e4 ,h4 ,richards4 cannot be too highly recommended4 ,teachers will find this little primer of #df pages invaluable with young children1 and with all who have had no previous training in chemistry4 ,as an admirable continuation of the work begun in these pages1 teachers are referred to ,professor ,shaler's 8,first ,book in ,geology40 ,in this our brief sketch of the geological agencies is amplified and beautifully illustrated2 and rarely have the wonderful stories of the river1 ocean-beach1 glacier1 and volcano been told so effectively4 ,in the chapter on the history of life on the globe the main outlines of historical geology are skillfully brought within the comprehension of beginners4 ,the directions to teachers are fully in accord with the modern methods and ideas1 and are a very valuable feature of the book4 ---------------------------------------- -------------------------------- ,,dynamical ,,geology4 ,illustration ,when we think of the ocean with its waves1 tides1 and currents1 of the winds1 and of the rain and snow1 and the vast net-work of rivers to which they give rise1 we realize that the energy or force manifested upon the earth's surface resides chiefly in the air and water--in the earth's fluid envelope and not in its solid crust4 ,and it would be an easy matter to show that1 with the exception of the tidal waves and currents1 which of course are due chiefly to the attraction of the moon1 nearly all this energy is merely the transformed heat of the sun4 ,now the air and water are two great geological agencies1 and therefore the geological effects which they produce are traceable back to the sun4 ,organic matter is another important geological agent2 but all are familiar with the generalization that connects the energy exhibited by every form of life with the sun2 and1 besides1 it is scarcely necessary to allude to the obvious fact that all animals and plants1 so far at least as any display of energy is concerned1 are merely differentiated portions of the earth's fluid envelope4 ,and so1 if space permitted1 it might be shown that1 with the exception of the tides1 nearly every form of force manifested upon the earth's surface has its origin in the sun4 ,of this trio of geological agencies operating upon the earth's surface and vitalized by the sun--water1 air1 and organic matter--the water is by far the most important1 and so it is common to call these collectively the aqueous agencies4 ,hence we have solar agencies and aqueous agencies as synonymous terms4 ,the aqueous agencies include1 on one side1 air and water1 or inorganic agencies2 and1 on the other1 animals and plants1 or organic agencies4 ,let us notice briefly the operation of these1 beginning with the air and water4 ,i4 ,,aqueous ,,agencies4 #a4 ,air and ,water1 or ,inorganic ,agencies4 ,,chemical ,,erosion4--,attention is invited first to the specimens numbered #a1 #b1 #c1 and #d4 ,no4 #a is a sound1 fresh piece of the rather common rock1 diabase2 and those who are acquainted with minerals will recognize that the light-colored grains in the rock are feldspar1 and the dark1 augite4 ,this specimen came from a depth in the quarry1 and has not been exposed to the action of the weather4 ,the second specimen differs from the first1 apparently1 as much as possible2 and yet1 except in being somewhat finer grained1 it was originally of precisely similar composition and appearance4 ,in fact1 it is a portion of the same rock1 but a weathered portion4 ,in this we can no longer recognize the feldspar and augite as such1 but both these minerals are very much changed1 while in the place of a strong1 hard rock we have an incoherent friable mass1 which is1 externally at least1 easily crushed to powder2 and with the next step in the weathering1 as we may readily observe in the natural ledges1 the rock is completely disintegrated1 forming a loose earth or soil4 ,we have two examples of such natural powders in the specimens numbered #c and #d2 and by washing these 7especially the finer one1 ,no4 #d7 with water1 we can prove that they consist of an impalpable substance which we may call clay1 and angular grains which we may call sand4 ,the sand-grains are really portions of the feldspar not yet entirely changed to clay4 ,thus we learn that the result of the exposure of this hard rock to the weather is that it is reduced to the condition of sand and clay4 ,what we mean especially by the weather are moisture and certain constituents of the air1 particularly carbon dioxide4 ,the action of the weather on the rocks is almost entirely chemical4 ,with a very few exceptions1 the principal minerals of which rocks are composed1 such as feldspar1 hornblende1 augite1 and mica1 are silicates1 i4e41 consist of silicic acid or silica combined with various bases1 especially aluminum1 magnesium1 iron1 calcium1 potassium1 and sodium4 ,now the silica does not hold all these bases with equal strength2 but carbon dioxide1 in the presence of moisture1 is able to take the sodium1 potassium1 calcium1 and magnesium away from the silica in the form of carbonates1 which1 being soluble1 are carried away by the rain-water4 ,the silicate of aluminum1 with more or less iron1 takes on water at the same time1 and remains behind as a soft1 impalpable powder1 which is common clay4 ,in the case of our diabase1 continued exposure to the weather would reduce the whole mass to clay4 ,but other rocks contain grains of quartz1 a hard mineral which cannot be decomposed1 and it always forms sand4 ,certain classes of rocks1 too1 such as the limestones and some iron-ores1 are completely dissolved by water holding carbon dioxide in solution1 and nothing is left to form soil1 except usually a small proportion of insoluble impurities like sand or clay4 ,let us see next how these agents of decay get at the rocks4 ,neither water nor air can penetrate the solid rock or mineral to any considerable extent1 so that practically the action is limited to surfaces1 and whatever multiplies surfaces must favor decomposition4 ,first1 we have the upper surface of the rock where it is bare1 but more especially where it is covered with soil1 for there it is always wet4 ,all rocks are naturally divided by joints into blocks1 which are frequently more or less regular1 and often of quite small size4 ,water and air penetrate into these cracks and decompose the surfaces of the blocks1 and thus the field of their operations is enormously extended4 ,these rock-blocks sometimes show very beautifully the progress of the decomposing agents from the outside inward by concentric layers or shells of rotten material1 which1 in the larger blocks1 often envelop a nucleus of the unaltered rock4 ,it is interesting to observe1 too1 that these concentric lines of decay cut off the angles of the original blocks1 so that the undecomposed nucleus1 when it is found1 is approximately spherical instead of cuboidal4 ,both these points are well illustrated by specimen ,no4 #b2 for although now nearly spherical1 it was originally perfectly angular1 and has become rounded by the peeling off1 in concentric layers1 of the decomposed material1 and in most cases several of these layers are distinctly visible1 like the coats of an onion4 ,but by stripping these off we should discover1 in all the larger balls at least1 a solid1 spheroidal nucleus1 while in the smaller balls the decomposition has penetrated to the centre4 ,in the rocks also we find many imperfect joints and minute cracks4 ,in cold countries these are extended and widened by the expansive power of freezing water1 and thus the surfaces of decomposition become constantly greater4 ,nearly all rocks suffer this chemical decomposition when exposed to the weather1 but in some the decay goes on much faster than in others4 ,diabase is one of the rocks which decay most readily2 while granite is1 among common rocks1 one of those that resist decay most effectually4 ,the caverns which are so large and numerous in most limestone countries are a splendid example of the solvent action of meteoric waters1 being formed entirely by the dissolving out of the limestone by the water circulating through the joint cracks4 ,the process must go on with extreme slowness at first1 when the joints are narrow1 and more rapidly as they are widened and more water is admitted4 ,we get some idea1 too1 of the magnitude of the results accomplished by these silent and unobtrusive agencies when we reflect that almost all the loose earth and soil covering the solid rocks are simply the insoluble residue which carbon dioxide and water cannot remove4 ,in low latitudes1 where a warm climate accelerates the decay of the rocks1 the soil is usually from #ej to #cjj feet deep4 ,,mechanical ,,erosion4--,on the edge of the land4--,let us trace next the mechanical action of water and air upon the land4 ,first we will consider the edge of the land1 where it is washed by the waves of the sea4 ,whoever has been on the shore must have noticed that the sand along the water's edge is kept in constant motion by the ebb and flow of the surf4 ,where the beach is composed of gravel or shingle the motion is evident to the ear as well as the eye2 and when the surf is strong1 the rattling and grinding of the pebbles as they are rolled up and down the beach develops into a roar4 ,the constant shifting of the grains of sand1 pebbles1 and stones is1 of course1 attended by innumerable collisions1 which are the cause of the noise4 ,now it is practically impossible1 as we may easily prove by experiment1 to knock or rub two pieces of stone together1 at least so as to produce much noise1 without abrading their surfaces2 small particles are detached1 and sand and dust are formed4 ,that this abrasion actually occurs in the case of the moving sand is most beautifully shown by the sandblast4 ,we are to conclude1 then1 that every time a pebble1 large or small1 is rolled up or down the beach it becomes smaller1 and some sand and dust or clay are formed which are carried off by the water4 ,but what are the pebbles originally8 ,this question is not difficult4 ,a little observation on the beach shows us that the pebbles are not all equally round and smooth1 but many are more or less angular4 ,and we soon see that it is possible to select a series showing all gradations between the most perfectly rounded forms and angular fragments of rock that are only slightly abraded on the corners4 ,the three principal members of such a series are shown in specimens #e1 #f1 and #g from the beach on ,marblehead ,neck2 but equally instructive specimens can be obtained at many other points on our coast4 ,it is also observable that the well-rounded pebbles are much smaller on the average than the angular blocks4 ,from these facts we draw the legitimate inference that the pebbles were all originally angular1 and that the same abrasion which diminishes their size makes them round and smooth4 ,a little reflection1 too1 shows that the rounding of the angular fragments is a natural and necessary result of their mutual collisions2 for the angles are at the same time their weakest and most exposed points1 and must wear off faster than the flat or concave surfaces4 ,having traced each pebble back to a larger angular rock-fragment1 the question arises1 ,whence come these angular blocks8 ,behind our gravel-beach1 or at its end1 we have usually a cliff of rocks4 ,as we approach this it is distinctly observable that the angular pebbles are more numerous1 larger1 and more angular2 and a little observation shows that these are simply the blocks produced by jointing1 and that the cliff is entirely composed of them4 ,in other words1 our cliff is a mass of natural masonry1 which chemical agencies1 the frost1 and the sea are gradually disintegrating and removing4 ,as soon as the blocks are brought within reach of the surf their mutual collisions make them rounder and smaller2 and small round pebbles1 sand1 and clay are the final result4 ,for a more complete account of the formation of pebbles1 teachers are referred to the first or introductory number of this series of guides1 by ,prof4 ,hyatt1 8,about ,pebbles40 ,where the waves can drive the shingle directly against the base of the cliff1 this is gradually ground away in the same manner as the loose stones themselves1 sometimes forming a cavern of considerable depth1 but always leaving a smooth1 hard surface1 which is very characteristic1 and contrasts strongly with the upper portion of the cliff1 which is acted on only by the rain and frost4 ,a good example of such a pebble-carved cliff may be seen behind the beach on the sea-ward side of ,marblehead ,neck4 ,the sea acts within very narrow limits vertically1 a few feet or a few yards at most2 but the coast-lines of the globe 7including inland lakes and seas7 have an aggregate length of more than #aej1jjj miles4 ,hence it is easy to see that the amount of solid rock ground to powder in the mill of the ocean-beach annually must be very considerable4 ,,mechanical ,,erosion4--,on the surface of the land4--,i next ask attention to the mechanical action of water upon the surface of the land4 ,it is a familiar fact that after heavy rains the roadside rills carry along much sand and clay 7which we know have been produced by the previous action of chemical forces71 and also frequently small pebbles or gravel4 ,it is easy to show that in all important respects the rill differs in size only from brooks and rivers2 and the former afford us fine models of the systems of valleys worn out during the lapse of ages by rivers4 ,the turbidity of rivers is often very evident1 and in shallow streams we can sometimes see the pebbles rolled along by the current4 ,now here1 just as on the beach1 the collisions of rock-fragments are attended by mutual abrasion1 sand and clay are formed1 and the fragments become smaller and rounder4 ,our series of pebbles from the beach might be matched perfectly among the river-gravel4 ,in mountain streams especially we may often observe that pebbles of a particular kind of rock become more numerous1 larger1 and more angular as we proceed up stream1 until we reach the solid ledge from which they were derived1 showing the same gradation as the beach pebbles when followed back to the parent cliff4 ,the pebbles1 however1 not only grind each other1 but also the solid rocks which form the bed of the streams in many places1 and these are gradually worn away4 ,when the rocky bed is uneven and the water is swift1 pebbles collect in hollows where eddies are formed1 by which they are kept whirling and turning1 and the hollow is deepened to a pot-hole1 while the pebbles1 the river's tools1 are worn out at the same time4 ,by these observations we learn not only that running water carries away sand and clay already formed1 but that it also has great power of grinding down hard rocks to sand and clay4 ,of course the pulverized rock always moves in the same direction as the stream which carries it2 and1 in a certain sense1 all streams run in one direction1 viz41 toward the sea4 ,therefore the constant tendency of the rain falling upon the land is to break up the rocks by chemical and mechanical action and transport the dbris to the sea4 ,rivers1 as we all know1 are continually uniting to form larger and larger streams2 and thus the drainage of a wide area sometimes1 as in the case of the ,mississippi ,valley1 reaches the sea through a single mouth4 ,by careful measurements made at the mouth of the ,mississippi it has been shown that the #bj1jjj1jjj1jjj1jjj cubic feet of water discharged into the ,gulf of ,mexico annually carries with it no less than #g1ejj1jjj1jjj cubic feet of sand1 clay1 and dissolved mineral matter2 and this1 spread over the whole ,mississippi basin1 would form a layer a little more than #a_/ejjj of a foot in thickness4 ,so that we may conclude that the surface of the continent is being cut down on the average about one foot in five thousand years4 ,we can only allude in passing to the very important geological action of water in the solid state1 as in glaciers and icebergs4 ,the moisture precipitated from the atmosphere1 and falling as rain1 makes ordinary rivers2 but falling in the form of snow in cold regions1 where more snow falls than is melted1 the excess accumulates and is gradually compacted to ice1 which1 like water1 yields to the enormous pressure of its own mass and flows toward lower levels4 ,when the ice-river reaches the sea it breaks off in huge blocks1 which float away as icebergs4 ,moving ice1 like moving water1 is a powerful agent of erosion2 and the glacial marks or scratches observable upon the ledges everywhere in the ,northern ,states and ,canada attest the magnitude of the ice-action at a comparatively recent period4 ,we have already noticed incidentally the powerful disintegrating action of water where it freezes in the joints and pores of the rocks2 and it is probable that it thus facilitates the destruction of the rocks in cold countries nearly as much as the higher temperature and greater rain-fall do in warm countries4 ,our observations up to this point show us that erosion1 by which we mean the breaking up by chemical and mechanical action of the rocks of the land and the transportation of the dbris into the sea1 is one great result accomplished by the inorganic aqueous agencies4 ,,mechanical ,,deposition4--,next let us notice what becomes of all this vast amount of clay1 sand1 and gravel after it is washed into the ocean4 ,by taking up a glass of turbid water from our roadside rill1 and observing that as soon as the water is undisturbed the sand and clay begin to settle1 we learn that the solid matter is held in suspension by the motion of the water4 ,but it does not remain in suspension long after being washed into the sea1 for otherwise the sea would1 in the course of time1 become turbid for long distances from shore2 and it is a well-known fact that the sea-water is usually clear and free from sensible turbidity close along shore and even near the mouths of large rivers1 while at a distance of only #ej or #ajj miles we find the transparency of the central ocean4 ,putting these facts together1 we see that the ocean1 nothwithstanding the ceaseless and often violent undulations of its surface1 must be as a whole a vast body of still water2 and to the reflecting mind the almost perfect tranquillity of the ocean is one of its most impressive features4 ,for it is in striking contrast1 in this respect1 with the more mobile aerial ocean above it4 ,we have got hold1 now1 of two facts of great geological importance3 7#a7 ,the dbris washed off the land by waves and rivers into the still water of the ocean very soon settles to the bottom2 and 7#b7 it nearly all settles on that part of the ocean-floor near the land4 ,and now we have in view the second great office of the inorganic aqueous agencies1--deposition1 the counterpart or complement of erosion4 ,the land is the great theatre of erosion and the sea of deposition2 the rocks which are constantly wasting away on the former are as constantly renewed in the latter4 ,we will now observe the process of deposition a little more closely4 ,each of these two bottles contains the same amount of fine yellow clay1 but in one the water is fresh1 and in the other it is salt4 ,at the beginning of the lesson1 as you may have observed1 ,i brought the clay in both bottles into suspension by violent agitation1 and since then they have remained undisturbed4 ,the main point is that the salt water has become quite clear1 while the fresh water is still distinctly turbid1 showing that the salt favors the rapid deposition of the clay4 ,at the second lecture1 a week later1 these two bottles1 yet undisturbed1 were exhibited1 and the fresh water seen to be still sensibly turbid4 ,the fact is1 the clay is not held in suspension wholly by the motion of the water2 but1 just as in the case of dust in the atmosphere1 a small portion of the medium is condensed around or adheres to each solid particle1 i4e41 each clay particle in our experiment has an atmosphere of water which moves with it and buoys it up4 ,now the effect of the salt is to diminish the adhesion of the water to the particles1 i4e41 to diminish their atmospheres1 and consequently their buoyancy4 ,the diminished adhesion of the salt water is well shown by the smaller drops which it forms on a glass rod4 ,the geological importance of this principle is very great2 for it is undoubtedly largely to the saltness of the sea that we owe its transparency1 and the fact that the fine1 clayey sediment from the land1 like the coarse1 is deposited near the shore4 ,this bottle of fresh water contains some fine gravel1 coarse sand1 fine sand1 and clay4 ,by agitating the water1 all this material is brought into suspension4 ,now1 suddenly placing the bottle in a state of rest1 we observe that the gravel falls to the bottom almost instantly1 followed quickly by the coarse sand1 and very soon afterward by the fine sand2 and then there appears to be a pause1 the fine particles of clay all remain in suspension2 but finally1 when the water is quite motionless1 they begin to settle2 they fall very slowly1 however1 and the water will not be clear for hours4 ,this is a very instructive experiment4 ,we learn from it3 ,first1 that the power of the water to hold particles in suspension is inversely proportional to the size of the particles2 ,second1 that all materials deposited in water are assorted according to size2 ,third1 and this is one of the most important facts in geology1 all water-deposited sediments are arranged in horizontal layers1 i4e41 are stratified4 ,and we have now traced to its conclusion1 though very briefly1 the process of the formation of one great division of stratified rocks1--the mechanically-formed or fragmental rocks4 ,these are so called because the clay1 sand1 and gravel are1 in every instance1 fragments of pre-existing rocks2 and because the formation1 transportation1 and especially the deposition of these fragments1 are the work chiefly or entirely of mechanical forces4 ,,chemical ,,deposition4--,it is a well-known fact that the sea holds in solution vast amounts of common salt as well as many other substances2 and analyses of river-waters show that dissolved minerals derived from the chemical decomposition of the rocks of the land are being constantly carried into the sea4 ,portions of the sea which are cut off from the main body1 and which are gradually drying up1 like the ,great ,salt ,lake1 ,dead ,sea1 and ,caspian ,sea1 become saturated solutions of the various dissolved minerals1 and these are slowly deposited4 ,this process is very nicely illustrated along our shores in summer1 where1 during storms1 salt-water spray is thrown above the reach of the tides1 and1 collecting in hollows in the rocks1 gradually dries up1 leaving behind a crust of salt4 ,when the sea lays down matter which it held in suspension1 we call the process mechanical deposition1 and the result is mechanically-formed rocks4 ,but when it lays down matter which it held in solution1 we call the process chemical deposition1 and the result is chemically-formed rocks4 ,the principal substances which the sea deposits chemically are common salt1 forming beds of rock-salt2 sulphate of calcium1 forming beds of gypsum2 carbonate of calcium1 forming beds of limestone2 and the double carbonate of calcium and magnesium1 forming beds of dolomite4 ,inorganic deposition1 like inorganic erosion1 is both chemical and mechanical4 #b4 ,animals and ,plants1 or ,organic ,agencies4 ,we turn now to the consideration of the organic agencies4 ,and ,i will merely allude in passing to the vast importance of the fossil organic remains found in the stratified rocks as marks by which to determine the relative ages of the formations4 ,as regards the destruction of rocks--erosion--plants and animals are almost powerless2 but in the role of rock-makers they play a very important part1 being very efficient agents of deposition4 ,,formation ,,of ,,coals ,,and ,,bitumens4--,specimen ,no4 #h is an example of peat from the vicinity of ,boston2 but just as good specimens may be obtained in thousands of places in this and other ,states4 ,the general physical conditions under which peat is formed are familiar facts4 ,we require simply low1 level land1 covered with a thin sheet of water and abundant vegetation2 in other words1 a marsh or swamp4 ,if plants decay on the dry land1 the decomposition is complete2 they are burned up by the oxygen of the air to carbon dioxide and water just as surely as if they had been thrown into a furnace1 though less rapidly1 and nothing is returned to the soil but what had been taken from it by the plants during their growth4 ,but if the plants decay under water1 as in a peat-marsh or bog1 the decay is incomplete1 and most of the carbon of the wood is left behind4 ,now1 if this incomplete combustion of vegetable tissues takes place in a charcoal-pit1 where the wood is out of contact with air from being covered with earth1 we call the carbonaceous product charcoal2 but if under the water of a marsh1 in ,nature's laboratory1 we call the product peat4 ,peat is simply a natural charcoal2 and1 just as in ordinary charcoal1 its vegetable origin is always perfectly evident4 ,but when the deposit becomes thicker1 and especially when it is buried under thick formations of other rocks1 like sand and clay1 the great pressure consolidates the peat2 it becomes gradually more mineralized and shining1 shows the vegetable tissues less distinctly1 becomes more nearly pure carbon1 and we call it in succession lignite1 bituminous coal1 and anthracite4 ,this is1 briefly1 the way in which all varieties of coal1 as well as the more solid kinds of bitumen1 like asphaltum1 are formed4 ,but the lighter forms of bitumen1 such as petroleum and naphtha1 are derived mainly1 if not entirely1 from the partial decomposition of animal tissues4 ,these1 it is well known1 decay much more readily than vegetable tissues2 and the water of an ordinary marsh or lake contains sufficient oxygen for their complete and rapid decomposition4 ,in the deeper parts of the ocean1 however1 the conditions are very different1 for recent researches have shown1 contrary to the old idea1 that the deep sea holds an abundant fauna4 ,all grades of animal life1 from the highest to the lowest1 have need of a constant supply of oxygen4 ,on the land vegetation is constantly returning to the air the oxygen consumed by animals1 but in the abysses of the ocean vegetable life is scarce or wanting2 and hence it must result that over these greater than continental areas countless myriads of animals are living habitually on short rations of oxygen1 and in water well charged with carbon dioxide1 the product of animal respiration4 ,as a consequence1 when these animals die their tissues do not find the oxygen essential for their perfect decomposition1 and in the course of time become buried1 in a half-decayed state1 in the ever-increasing sediments of the ocean-floor4 ,it is important to observe that an abundance of organic matter decaying under water is not the only condition essential to the formation of beds of coal and bitumen2 for this condition is realized in the luxuriant growth of sea-weeds fringing the coast in every quarter of the globe2 and yet coals and bitumens are rarely of sea-shore origin4 ,these organic products1 even under the most favorable circumstances1 accumulate with extreme slowness2 far more slowly1 as a rule1 than the ordinary mechanical sediments1 like sand and clay1 with which they are mixed1 and in which they are often completely lost4 ,consequently1 although the deposition of the carbonized remains of plants and animals is taking place in nearly all seas1 lakes1 and marshes1 it is only in those places where there is little or no mechanical sediment that they can predominate so as to build up beds pure enough to be called coal or bitumen4 ,in all other cases we get merely more or less carbonaceous sand or clay4 ,now these especially favorable localities will manifestly not be often found along the seashore1 where we have strewn the sand and clay brought down by rivers or washed off the land directly by the ever-active surf2 but they must exist in the central portions of the ocean1 where there is almost no mechanical sediment and yet an abundance of life1 and in swamps and marshes1 where there is scarcely sufficient water to cover the vegetation1 and no waves or currents to wash down the soil from the surrounding hills4 ,,formation ,,of ,,iron-,,ores4--,the iron-ores are another class of rocks which are formed only through the agency of organic matter4 ,iron is an abundant and wide-spread element in the earth's crust1 and1 but for the intervention of life1 we might say that1 while there is iron everywhere1 there is not much of it in any one place1 since it is originally very thinly diffused4 ,all rocks and soils contain iron1 but it is mainly in the form of the peroxide1 in which state it is entirely insoluble1 and hence cannot be soaked out of the soil by the rain-water and concentrated by the evaporation of the water at lower levels in ponds and marshes1 as a soluble substance like salt would be4 ,if carried off with the sand and clay1 by the mechanical action of water1 it remains uniformly mixed with them1 and there is no tendency to its separation and concentration so as to form a true iron-ore4 ,but what water cannot do alone is accomplished very readily when the water is aided by decaying organic matter1 which is always hungry for oxygen1 being1 in the language of the chemist1 a powerful reducing agent4 ,the soil1 in most places1 has a superficial stratum of vegetable mould or half-decayed vegetation4 ,the rainwater percolates through this and dissolves more or less of the organic matter1 which is thus carried down into the sand and clay beneath and brought in contact with the ferric oxide1 from which it takes a certain proportion of oxygen1 reducing the ferric to the ferrous oxide4 ,at the same time the vegetation is burned up by the oxygen thus obtained1 forming carbon dioxide1 which immediately combines with the ferrous oxide1 forming carbonate of iron1 which1 being soluble under these conditions1 is carried along by the water as it gradually finds its way by subterranean drainage to the bottom of the valley and emerges in a swamp or marsh4 ,here one of two things will happen3 ,if the marsh contains little or no decaying vegetation1 then as soon as the ferrous carbonate brought down from the hills is exposed to the air it is decomposed1 the carbon dioxide escapes1 and the iron1 taking on oxygen from the air1 returns to its original ferric condition2 and being then quite insoluble1 it is deposited as a loose1 porous1 earthy mass1 commonly known as bog-iron-ore1 which becomes gradually more solid and finally even crystalline through the subsequent action of heat and pressure4 ,when first deposited1 the ferric oxide is combined with water or hydrated1 and is then known as limonite 7specimen ,no4 #ab72 at a later period the water is expelled1 and we call the ore hematite 7specimen ,no4 #ac72 and at a still later age it loses part of its oxygen1 becomes magnetic and more crystalline1 and is then known as magnetite 7specimen ,no4 #ad74 ,thus it is seen that the iron-ores1 as we pass from bog-limonite to magnetite1 form a natural series similar to and parallel with that afforded by the coals as we pass from peat to graphite4 ,if the drainage from the hills is into a marsh containing an abundance of decaying vegetation1 i4e41 if peat is forming there1 the ferrous carbonate1 in the presence of the more greedy organic matter1 will be unable to obtain oxygen from the air2 and as the evaporation of the water goes on1 it will sooner or later become saturated with this salt1 and the latter will be deposited4 ,here we find an explanation of a fact often observed by geologists1 viz41 that the carbonate iron-ores are usually associated with beds of coal4 ,the formation of the iron-ores1 like that of the coals and bitumens1 is a slow process2 and the ores1 like the coals1 etc41 will be pure only where there is a complete absence of mechanical sediment1 a condition that is realized most nearly in marshes4 ,,formation ,,of ,,limestone1 ,,diatomaceous ,,earth1 ,,etc4--,marine animals take from the sea-water certain mineral substances1 especially silica and carbonate of calcium1 to form their skeletons4 ,silica is used only by the lowest organisms1 such as ,radiolaria1 ,sponges1 and the minute unicellular plants1 ,diatoms4 ,the principal animals secreting carbonate of calcium are ,corals and ,mollusks4 ,these hard parts of the organisms remain undissolved after death2 and over portions of the ocean-floor where there is but little of other kinds of sediment they form the main part of the deposits1 and in the course of ages build up very extensive formations which we call diatomaceous earth or tripolite1 if the organisms are siliceous1 or limestone if they are calcareous4 ,a very satisfactory account of the formation of limestone on a stupendous scale by the polyps in coral reefs and islands is contained in ,no4 ,,iv4 of this series of guides4 ,the rocks here considered may be1 and1 as we have already seen1 sometimes are1 deposited in a purely chemical way1 without the aid of life2 and it is important to observe that in no case do the organisms make the silica and carbonate of calcium of their skeletons1 but they simply appropriate and reduce to the solid state what exists ready made in solution in the sea-water4 ,these minerals1 and others1 as we know1 are produced by the decomposition of the rocks of the land1 and are being constantly carried into the sea by rivers2 and1 if there were no animals in the sea1 these processes would still go on until the sea-water became saturated with these substances1 when their precipitation as limestone1 etc41 would necessarily follow4 ,hence it is clear that all the animals do is to effect the precipitation of certain minerals somewhat sooner than it would otherwise occur2 so that from a geological standpoint the differences between chemical and organic deposition are not great4 ,this section of our subject may be summarized as follows3 ,animals and plants contribute to the formation of rocks in three distinct ways3-- #a4 ,during their growth they deoxidize carbon dioxide and water1 and reduce to the solid state in their tissues carbon and the permanent gases oxygen1 hydrogen1 and nitrogen2 and after death1 through the accumulation of the half-decayed tissues in favorable localities1--marshes1 etc41--these elements are added to the solid crust of the earth in the form of coal and bitumen4 #b4 ,during the decomposition1 i4e41 oxidation1 of the organic tissues1 the iron existing everywhere in the soil is partially deoxidized1 and1 being thus rendered soluble1 is removed by rain-water and concentrated in low places1 forming beds of iron-ore4 #c4 ,through the agency of marine organisms1 certain mineral substances are being constantly removed from the sea-water and deposited upon the ocean floor1 forming various calcareous and siliceous rocks4 ,i now bring our study of the aqueous or superficial agencies to a conclusion by noting once more that the great geological results accomplished by air1 water1 and organic matter or life are3 7#a7 ,erosion1 or the wearing away of the surface of the land2 and 7#b7 ,deposition1 or the formation from the dbris of the eroded land of two great classes of stratified rocks1--the mechanically formed or fragmental rocks1 and the chemically and organically formed rocks4 ,,ii4 ,,igneous ,,agencies4 ,we pass next to a very brief consideration of operations that originate below the earth's surface4 ,the records of deep mines and artesian wells show that the temperature of the ground always increases downwards from the surface2 and the much higher temperatures of hot springs and volcanoes show that the heat continues to increase to a great depth1 and is not a merely superficial phenomenon4 ,the observed rate of increase is not uniform1 but it seldom varies far from the average1 which is about #a ,fahr4 per #ec feet of vertical descent1 or1 in round numbers1 #ajj per mile4 ,this rate1 if continued1 would give a very high temperature at points only a few miles below the surface2 and until within a few years the idea was generally accepted by geologists that the increase of temperature is sensibly uniform for an indefinite distance downward2 that in the central regions of the earth the temperature is far higher than anything we can conceive1 and that everywhere below a depth of #bj to #dj miles the temperature is above the fusing-point of all rocks2 and hence that the earth is an incandescent liquid globe covered by a thin shell or crust of cold1 solid rock4 ,our limited space will not permit us to enter into a discussion of the condition of the earth's interior1 and ,i will merely point out in a few sentences the position occupied by geologists at the present time4 ,the reasoning of ,thompson has shown that the temperature cannot increase downward at a uniform rate1 but at a constantly and rapidly diminishing rate2 and that everywhere below a depth of #cjj miles the temperature is probably sensibly the same1 and nowhere1 probably1 above #hjjj to #aj1jjj ,fahr4 ,unlike water1 all rocks contract on solidifying and expand on melting1 and consequently the high pressures to which they are subjected in the earth's interior--#aj1jjj1jjj to #bj1jjj1jjj pounds per square inch--must raise their fusing-points enormously1 and the probabilities are that they are solid1 in spite of the high temperature4 ,but ,thompson and ,darwin have shown us farther that the phenomena of the oceanic tides could not be what they are known to be if the earth were any less rigid than a globe of solid steel2 while ,hopkins has proved that the astronomical phenomena of precession and nutation could not be what they are if the earth's crust were less than #hjj or #ajjj miles thick4 ,putting these considerations together1 geologists are almost universally agreed that1 while the earth has an incandescent interior1 it is still continuously solid from centre to circumference1 with the exception of a thin plastic stratum at a depth not exceeding #dj or #ej miles1 which forms the seat of volcanic action4 ,the earth is not only a very hot body1 but it is rotating through almost absolutely cold space1 and therefore must be a cooling body4 ,but1 except at the very beginning of the cooling1 the loss of heat has gone on almost entirely from the interior2 and since cooling means contraction1 the heated interior must be constantly tending to shrink away from the cold external crust4 ,of course no actual separation between the crust and interior or nucleus can take place1 but there is no doubt that the crust is left unsupported to a certain extent1 and it must then behave like an arch with a radius of #djjj miles1 and the result is an enormous horizontal or tangential pressure4 ,this lateral pressure in the earth's crust is one of the most important and most generally accepted facts in geology1 and lies at the bottom of many geological theories4 ,according to what seems to me to be the most probable theory of the origin of continents and ocean-basins1 they are broad upward and downward bendings or arches into which the crust is thrown by the tangential pressure4 ,finally1 the strain becomes great enough to crush the crust along those lines where it is weakest4 ,when the crust is thus mashed up by horizontal pressure1 a mountain range is formed1 the crust becomes enormously thicker1 and a weak place becomes a strong one4 ,during the formation of mountains the stratified rocks1 which were originally horizontal1 are thrown into folds or arches1 and tipped up at all possible angles2 they are fractured and faults produced2 and by the immense pressure the structure known as slaty cleavage is developed4 ,in fact1 a vast amount and variety of structures are produced during the growth of a mountain range4 ,these great earth-movements are not always perfectly smooth and steady1 but they are accompanied by slipping or crushing now and then2 and1 as a result of the shock thus produced1 a swift vibratory movement or jar1 which we know as an earthquake1 runs through the earth's crust4 ,extensive fissures are also formed1 opening down to the regions where the rocks are liquid or plastic1 and through these the melted rocks flow up to or toward the surface4 ,that portion which flows out on the surface builds up a volcanic cone1 while that which cools and solidifies below the surface1 in the fissures1 forms dikes4 ,thus among the igneous or eruptive rocks we have two great classes1--the dike rocks and the volcanic rocks4 ,it is important to observe that all these subterranean operations--the formation of continents1 of mountain-ranges with all their attendant phenomena of folds1 faults and cleavage1 and every form and phase of earthquake and volcanic activity--depend upon or originate in the interior heat of the earth4 ,and over against the superficial or aqueous agencies1 originating in the solar heat and producing the stratified or sedimentary rocks1 we set the subterranean or igneous agencies originating in the central heat1 and producing the unstratified or eruptive rocks4 ---------------------------------------- -------------------------------- ,,structural ,,geology4 ,illustration ,in geology1 just as in biology1 there are two ways of studying structure1--the small way and the large way4 ,in the case of an organism1 we may select a single part or organ1 and1 disregarding its external form and relations to other parts1 observe its composition and minute structure1 the various forms and arrangements of the cells1 etc4 ,this is histology1 and it is the complement of that larger method of studying structure which is ordinarily understood by anatomy4 ,the divisions of structural geology corresponding to histology and anatomy are lithology and petrology4 ,lithology is an in-door science2 we use the microscope largely1 and work with hand specimens or thin sections of the rocks1 observing the composition and those small structural features which go under the general name of texture4 ,in petrology1 on the other hand1 we consider the larger kinds of rock-structure1 such as stratification1 jointing1 folds1 faults1 cleavage1 etc42 and it is essentially an out-door science1 since to study it to the best advantage we must have1 not hand specimens1 but ledges1 cliffs1 railway-cuttings1 gorges1 and mountains4 ,,lithology4 ,a rock is any mineral1 or mixture of minerals1 occurring in masses of considerable size4 ,this distinction of size is the only one that can be made between rocks and minerals1 and that is very indefinite4 ,a rock1 whether composed of one mineral or several1 is always an aggregate2 and therefore no single crystal or mineral-grain can properly be called a rock4 ,before proceeding to study particularly the various kinds of rocks1 a little more preliminary work should be done4 ,as already intimated1 the more important characteristics of rocks may be grouped under two general heads1--composition and texture4 ,composition of ,rocks4 ,rocks are properly defined as large masses or aggregates of mineral matter1 consisting in some cases of one and in other cases of several mineral species4 ,hence it is clear that the composition of rocks is of two kinds3 chemical and mineralogical2 for the various chemical elements are first combined to form minerals1 and then the minerals are combined to form rocks4 ,of course those minerals and elements which can be described as principal or important rock-constituents must be the common minerals and elements4 ,now it is a very important and convenient fact that although chemists recognize about sixty-five elementary substances1 and these are combined to form nearly one thousand mineral species1 yet both the common elements and the common minerals are few in number4 ,so that1 although it is very desirable and even necessary for the student of lithology to know something of chemistry and mineralogy1 it by no means follows that he or she must be master of those sciences4 ,a knowledge of the chemical and physical characteristics of a few common minerals is all that is absolutely essential1 though it may be added that an excess of wisdom in these directions is no disadvantage4 ,chemical ,composition of ,rocks4 ,the elementary substances of which rocks are chiefly composed1 which make up the main mass of the earth so far as we are acquainted with it1 number only fourteen3-- ,non-,metallic or ,acidic ,elements4--,oxygen1 silicon1 carbon1 sulphur1 chlorine1 phosphorus1 and fluorine4 ,metallic or ,basic ,elements4--,aluminum1 magnesium1 calcium1 iron1 sodium1 potassium1 and hydrogen4 ,the elements are named in each group in about the order of their relative abundance2 and to give some idea of the enormous differences in this respect it may be stated that two of the elements--oxygen and silicon--form more than half of the earth's crust4 ,silicon1 calcium1 and fluorine1 although exceedingly abundant1 are also very difficult to obtain in the free or uncombined state1 and specimens large enough to exhibit to a class would be very expensive4 ,with these exceptions1 however1 examples of these common rock-forming elements are easily obtained4 ,my purpose in calling attention to this point is simply to suggest that the proper way to begin the study of minerals and rocks with children is to first familiarize them with the elements of which they are composed4 ,the most important thing to be known about any mineral is its chemical composition2 and when a child is told that a mineral--corundum1 for example--is composed of oxygen and aluminum1 he should have a distinct conception of the properties of each of those elements1 for otherwise corundum is for him a mere compound of names4 ,it is very important1 too1 if the pupil has not already studied chemistry1 that he should be led to some comprehension of the nature of chemical union and of the difference between a chemical compound and a mechanical mixture4 ,for this purpose a few simple experiments 7the details of which would be out of place here7 with the more common and familiar elements will be sufficient4 ,mrs4 ,richard's 8,first ,lessons in ,minerals0 should be introduced here4 ,mineralogical ,composition of ,rocks4 ,the fourteen elements named above are combined to form about fifty minerals with which the student of geology should be acquainted2 but not more than one-half of these are of the first importance4 ,it is desired to lay especial emphasis upon the importance of a perfect familiarity with these few common minerals4 ,there is nothing else in the whole range of geology so easily acquired which is at the same time so valuable2 for it is entirely impossible to comprehend the definitions of rocks1 or to recognize rocks certainly and scientifically1 unless we are acquainted with their constituent minerals4 ,with one or two exceptions1 these common rock-forming minerals may be easily distinguished by their physical characters alone1 so that their certain recognition is a matter of the simplest observation1 and entirely within the capacity of young children4 ,furthermore1 being common1 specimens of these minerals are very easily obtained1 so that there is no reason why teachers should not here adopt the best method and place a specimen of each mineral in the hands of each pupil4 ,typical examples1 large enough to show the characteristics well1 ought not to cost1 on the average1 over two cents apiece4 ,a ,,mineral is an inorganic body having theoretically a definite chemical composition1 and usually a regular geometric form4 ,,the ,,principal ,,characteristics ,,of ,,minerals4--,these may be grouped under the following general heads3-- 7#a7 ,composition1 7#b7 ,crystalline form1 7#c7 ,hardness1 7#d7 ,specific gravity1 7#e7 ,lustre1 7#f7 ,color and ,streak4 #a4 ,composition4--,this1 according to the definition of a mineral1 ought to be definite1 and expressible by a chemical formula4 ,when it is not so1 we usually consider that the mineral is partially decomposed1 or that we are dealing with a mixture of minerals4 ,it is well to impress upon the mind of the pupil the important fact that the more fundamental properties of the elements1 such as specific gravity and lustre1 are not lost when they combine1 but may be traced in the compounds4 ,in other words1 the properties of minerals are1 in a very large degree1 the average of the properties of the elements of which they are composed2 minerals in which heavy metallic elements predominate being heavy and metallic1 and vice versa4 ,to fully appreciate this point it is only necessary to compare a mineral like galenite--a common ore of lead1 and containing nearly #hg per cent4 of that heavy metal2 or hematite 7specimen #ac71 containing #gj per cent4 of another heavy metal1 iron--with quartz 7specimen #ae71 which is composed in nearly equal parts of oxygen and silicon1 two typical non-metallic elements4 ,many minerals contain water1 i4e41 are hydrated4 ,now water1 whether we consider the liquid or solid state1 is one of the lightest and softest of mineral constituents2 and it is a very important fact that hydrated minerals are invariably lighter and usually softer than anhydrous species of otherwise similar composition4 ,other striking illustrations of this principle will be pointed out in the descriptions of the minerals which follow4 #b4 ,crystalline form4--,a crystal is bounded by plane surfaces symmetrically arranged with reference to certain imaginary lines passing through its centre and called axes4 ,crystals of the same species are always constant in the angles between like planes1 while similar angles usually vary in different species2 so that each species has its own peculiar form4 8,besides external symmetry of form1 crystallization produces also regularity of internal structure1 and often of fracture4 ,this regularity of fracture1 or tendency to break or cleave along certain planes1 is called cleavage4 ,the surface afforded by cleavage is often smooth and brilliant 7see specimens #ag1 #ah1 and #ba71 and is always parallel with some external plane of the crystal4 ,it should be understood that the cleavage lamell are not in any sense present before they are made to appear by fracture40--7,dana47 ,crystals are arranged in six systems1 based upon the number and relations of the axes1 as follows3-- ,isometric ,system4--,three equal axes crossing at right angles4 ,example1 cube4 ,tetragonal ,system4--,two axes equal1 third unequal1 all crossing at right angles4 ,example1 square prism4 ,orthorhombic ,system4--,three unequal axes1 but intersections all at right angles4 ,example1 rhombic prism4 ,monoclinic ,system4--,three unequal axes1 one intersection oblique4 ,example1 oblique rhombic prism4 ,triclinic ,system4--,three unequal axes1 all crossing obliquely4 ,example1 oblique rhomboidal prism4 ,hexagonal ,system4--,three equal axes lying in one plane and intersecting at angles of #fj1 and a fourth axis crossing each of these at right angles and longer or shorter4 ,example1 hexagonal prism4 ,by the truncation and bevelment of the angles and edges of these fundamental forms a vast variety of secondary forms are produced4 ,the limits of the guide will not permit us to follow this topic farther2 but it may be added that for the proper elucidation of even the simpler crystalline forms the teacher should be provided with a set of wooden crystal models and ,dana's 8,text-,book of ,mineralogy40 ,the crystallization of a mineral may be manifested in two ways3 first1 by the regularity of its internal structure or molecular arrangement1 as shown by cleavage and the polarization of transmitted light2 and1 second1 by the regularity of external form which follows1 under favorable conditions1 as a necessary consequence of symmetry in the arrangement of the molecules4 ,when a mineral is entirely devoid of crystalline structure1 both externally and internally1 it is said to be amorphous4 ,perfect and distinct crystals are the rare exception1 most mineral specimens being simply aggregates of imperfect crystals4 ,in such cases1 and when the mineral is amorphous1 the structure of the mass may be3-- ,columnar or fibrous4 ,lamellar1 foliaceous1 or micaceous4 ,granular4--,when the grains or crystalline particles are invisible to the naked eye the mineral is called impalpable1 compact1 or massive4 ,and the external form of the mass may be3-- ,botryoidal1 having grape-like surfaces4 ,stalactitic1 forming stalactites or pendant columns4 ,amygdaloidal or ,concretionary1 forming separate globular masses in the enclosing rock4 ,dendritic1 branching or arborescent4 #c4 ,hardness4--,by the hardness of a mineral we mean the resistance which it offers to abrasion4 ,but hardness is a purely relative term1 calcite1 for example1 being hard compared with talc1 but very soft compared with quartz4 ,hence mineralogists have found it necessary to select certain minerals to be used as a standard of comparison for all others1 and known as the scale of hardness4 ,these are arranged at nearly equal intervals all the way from the softest mineral to the hardest1 as follows3-- ,scale of ,hardness4 7#a7 ,talc4 7#b7 ,gypsum4 7#c7 ,calcite4 7#d7 ,fluorite4 7#e7 ,apatite4 7#f7 ,orthoclase4 7#g7 ,quartz4 7#h7 ,topaz or ,beryl4 7#i7 ,corundum4 7#aj7 ,diamond4 ,if a mineral scratches calcite and is scratched by fluorite1 we say its hardness is between #c and #d1 perhaps #c4e2 if it neither scratches nor is scratched by orthoclase1 its hardness is #f2 and so on4 ,there are very few minerals harder than quartz1 and hence the first seven members of the scale are sufficient for all ordinary purposes2 and these are all included in the series of specimens accompanying this ,guide4 ,although it is desirable to be acquainted with the scale of hardness1 and to understand how to use it1 still the student will learn1 after a little practice1 that almost as good results may be obtained much more conveniently by the use of his thumb-nail and a good knife-blade or file4 ,talc and gypsum are easily scratched with the nail2 calcite and fluorite yield easily to the knife or file1 apatite with more difficulty2 while orthoclase is near the limit of the hardness of ordinary steel1 and quartz is entirely beyond it4 #d4 ,specific ,gravity4--,the specific gravity of a mineral1 by which we mean its weight as compared with the weight of an equal volume of water1 is determined by weighing it first in air and then in water1 and dividing the weight in air by the difference of the two weights4 ,minerals exhibit a wide range in specific gravity2 from petroleum1 which floats on water1 to gold1 which is nearly twenty times heavier than water4 ,although this is one of the most important properties of minerals1 yet1 being more difficult to measure than hardness1 it is less valuable as an aid in distinguishing species4 ,one can with practice1 however1 estimate the density of a mineral pretty closely by lifting it in the hand4 #e4 ,lustre4--,of all the properties of minerals depending on their relations to light the most important is lustre1 by which we mean the quality of the light reflected by a mineral as determined by the character or minute structure of its surface4 ,two kinds of lustre1 the metallic and vitreous1 are of especial importance2 in fact all other kinds are merely varieties of these4 ,the metallic lustre is the lustre of all true metals1 as copper and tin1 and characterizes nearly all minerals in which metallic elements predominate4 ,the vitreous lustre is best exemplified in glass1 but belongs to most minerals composed chiefly of non-metallic elements4 ,metallic minerals are always opaque1 but vitreous minerals are often transparent4 ,other kinds of lustre are the adamantine 7the lustre of diamond71 resinous1 pearly1 and silky4 ,when a mineral has no lustre1 like chalk1 it is said to be dull4 ,it should be made clear to children that lustre and color are entirely distinct and independent4 ,thus1 iron1 copper1 gold1 silver1 and lead are all metallic2 while white or colorless quartz1 black tourmaline1 green beryl1 red garnet1 etc41 are all vitreous4 ,generally speaking1 any color may occur with any lustre4 #f4 ,color and ,streak4--,the colors of minerals are of two kinds1--essential and non-essential4 ,by the essential color in any case we mean the color of the mineral itself in its purest state4 ,the non-essential colors1 on the other hand1 are chiefly the colors of the impurities contained in the minerals4 ,metallic minerals1 which are always opaque1 usually have essential colors2 but vitreous minerals1 which are always more or less transparent1 often have non-essential colors4 ,the explanation is this3 ,in opaque minerals we can only see the impurities immediately on the surface1 and these are1 as a rule1 not enough to affect its color2 but in diaphanous minerals we look into the specimen and see impurities below the surface1 and thus bring into view1 in many cases1 sufficient impurity so that its color drowns that of the mineral4 ,to prove this we have only to take any mineral 7serpentine is a good example in our series7 having a non-essential color1 and make it opaque by pulverizing it or abrading its surface1 when the non-essential color1 the color of the impurity1 immediately disappears2 just as water1 yellow with suspended clay1 becomes white when whipped into foam1 and thus made opaque4 ,what we understand by the streak of a mineral is its essential color1 the color of its powder2 and it is so called because the powder is most readily observed by scratching the surface of the mineral1 and thereby pulverizing a minute portion of it4 ,the streak and hardness are thus determined at the same time4 ,the streak of soft minerals is easily determined by rubbing them on any white surface of suitable hardness1 as paper1 porcelain1 or ,arkansas stone4 ,,essential ,,and ,,accessory ,,minerals4--,lithologists1 regarding minerals as constituents of rocks1 divide them into two great classes3 the essential and the accessory4 ,the essential constituents of a rock are those minerals which are essential to the definition of the rock4 ,for example1 we cannot properly define granite without naming quartz and orthoclase2 hence these are essential constituents of granite2 and if either of these minerals were removed from granite it would not be granite any longer1 but some other rock4 ,but other minerals1 like tourmaline and garnet1 may be indifferently present or absent2 it is granite still2 hence they are merely accidental or accessory constituents4 ,they determine the different varieties of granite1 while the essential minerals make the species4 ,this classification1 of course1 is not absolute1 for in many cases the same mineral forms an essential constituent of one rock and an accessory constituent of another4 ,thus1 quartz is essential in granite1 but accessory in diorite4 ,,principal ,,minerals ,,constituting ,,rocks4--,having studied in a general way the more important characteristics of minerals1 brief descriptions of the chief rock-forming species are next in order4 ,we will notice first and principally those minerals occurring chiefly as essential constituents of rocks4 #a4 ,graphite4--,essentially pure carbon1 though often mixed with a little iron oxide4 ,crystallizes in hexagonal system1 but usually foliated1 granular1 or massive4 ,hardness1 #a-b1 being easily scratched with the nail4 ,sp4 gr41 #b4a-b4c4 ,lustre1 metallic2 an exception to the rule that acidic elements have non-metallic or vitreous lustres4 ,streak1 black and shining 7see pencil-mark on white paper74 ,color1 iron-black4 ,slippery or greasy feel4 ,every black-lead pencil is a specimen of graphite4 ,specimen #i4 ,the different kinds of mineral coal are1 geologically1 as we have seen1 closely related to graphite1 but they are such familiar substances that they need not be described here4 #b4 ,halite 7common salt74--,chloride of sodium3 chlorine1 #fj4g2 sodium1 #ci4c2 #ajj4 ,isometric system1 usually forming cubes4 ,hardness1 #b4e1 a little harder than the nail4 ,sp4 gr41 #b4a-b4f4 ,lustre1 vitreous4 ,streak and color both white1 and hence color is essential4 ,often transparent4 ,soluble2 taste1 purely saline4 ,in specific gravity and lustre it is a good example of a mineral in which an acidic element predominates4 ,specimen #aa4 #c4 ,limonite4--,hydrous sesquioxide of iron3 oxygen1 #be2 iron1 #fj2 water1 #ae2 #ajj4 ,usually amorphous2 occurring in stalactitic and botryoidal forms1 having a fibrous structure2 and also concretionary1 massive1 and earthy 7yellow ochre74 ,hardness1 #e-e4e4 ,sp4 gr41 #c4f-d4 ,lustre1 vitreous or silky1 inclining to metallic1 and sometimes dull4 ,color1 various shades of black1 brown1 and yellow4 ,streak1 ochre-yellow2 hence color partly non-essential4 ,specimen #ab4 #d4 ,hematite4--,sesquioxide of iron3 oxygen1 #cj2 iron1 #gj2 #ajj4 ,hexagonal system1 in distinct crystals1 but usually lamellar1 granular1 or compact1--columnar1 botryoidal1 and stalactitic forms being common4 ,hardness1 #e4e-f4e2 good crystals are harder than steel4 ,sp4 gr41 #d4e-e4c4 ,lustre1 metallic1 sometimes dull4 ,color1 iron-black1 but red when earthy or pulverized 7red ochre74 ,streak1 red1 and color1 therefore1 mainly non-essential2 sometimes attracted by the magnet4 ,specimen #ac4 ,hematite has the same composition as limonite1 minus the water2 and by comparing the hardness and specific gravity of these two minerals we see that they are a good illustration of the principle that hydrous minerals are softer and lighter than anhydrous minerals of analogous composition4 ,limonite and hematite are two great natural coloring agents1 and almost all yellow1 brown1 and red colors in rocks and soils are due to their presence4 #e4 ,magnetite4--,protoxide and sesquioxide of iron3 oxygen1 #bg4f2 iron1 #gb4d2 #ajj4 ,isometric system1 usually in octahedrons or dodecahedrons4 ,most abundant variety is coarsely to finely granular1 sometimes dendritic4 ,hardness1 #e4e-f4e1 same as hematite4 ,sp4 gr41 #d4i-e4b4 ,lustre1 metallic4 ,color and streak1 iron-black1 and hence color essential4 ,strongly magnetic2 some specimens have distinct polarity1 and are called loadstones4 ,specimen #ad4 ,the three iron-oxides just described--limonite1 hematite1 and magnetite--are all important ores of iron1 and form a well-marked natural series4 ,thus limonite is never1 hematite is usually1 and magnetite is always1 crystalline4 ,again1 limonite with #fj per cent4 of iron is never magnetic1 hematite with #gj per cent4 is sometimes magnetic1 while magnetite with #gb4d per cent4 is always magnetic4 ,as the iron increases so does the magnetism4 ,we have here an excellent illustration of the principle that the properties of the elements can be traced in those minerals in which they predominate4 ,iron is the only strongly magnetic element3 magnetite contains more iron than any other mineral1 and it is the only strongly magnetic mineral4 ,these three iron-ores are easily distinguished from each other by the color of their powders or streak1--limonite yellow1 hematite red1 and magnetite black1--and from all other common minerals by their high specific gravity4 #f4 ,quartz4--,oxide of silicon or silica3 oxygen1 #ec4cc2 silicon1 #df4fg2 #ajj4 ,hexagonal system4 ,the most common form is a hexagonal prism terminated by a hexagonal pyramid4 ,also coarsely and finely granular to perfectly compact1 like flint2 the compact or cryptocrystalline varieties often assuming botryoidal1 stalactitic1 and concretionary forms4 ,it has no cleavage1 but usually breaks with an irregular1 conchoidal fracture like glass4 ,hardness1 #g1 being ,no4 #g of the scale2 scratches glass easily4 ,sp4 gr41 #b4e-b4h4 ,lustre1 vitreous4 ,pure quartz is colorless or white1 but by admixture of impurities it may be of almost any color4 ,streak always white or light colored4 ,quartz is usually1 as in specimen #ae1 transparent and glassy1 but may be translucent or opaque4 ,it is almost absolutely infusible and insoluble4 ,the varieties of quartz are very numerous1 but they may be arranged in two great groups3-- #a4 ,phenocrystalline or vitreous varieties1 including rock-crystal1 amethyst1 rose quartz1 yellow quartz1 smoky quartz1 milky quartz1 ferruginous quartz1 etc4 #b4 ,cryptocrystalline or compact varieties1 including chalcedony1 carnelian1 agate1 onyx1 jasper1 flint1 chert1 etc4 ,only three varieties1 however1 are of any great geological importance2 these are3 common glassy quartz 7spec4 #ae71 flint 7spec4 #af71 and chert4 ,quartz is one of the most important constituents of the earth's crust1 and it is also the hardest and most durable of all common minerals4 ,we have already observed 7p4 #ab7 that it is entirely unaltered by exposure to the weather2 i4e41 it cannot be decomposed2 and1 being very hard1 the same mechanical wear which1 assisted by more or less chemical decomposition1 reduces softer minerals to an impalpable powder or clay1 must leave the quartz chiefly in the form of sand and gravel4 ,this agrees with our observation that sand 7spec4 #cj71 especially1 is usually merely pulverized quartz4 ,opal is a mineral closely allied to quartz1 and may be mentioned in this connection4 ,it is of similar composition1 but contains from #e to #bj per cent4 of water1 and is decidedly softer and lighter4 ,hardness1 #e4e-f4e2 sp4 gr41 #a4i-b4c4 #g4 ,gypsum4--,hydrous sulphate of calcium3 sulphur trioxide 7,,so71 #df4e2 lime 7,ca,o71 #cb4f2 water 7,h,o71 #bj4i2 #ajj4 ,monoclinic system4 ,often in distinct rhombic crystals2 also foliated1 fibrous1 and finely granular4 ,hardness1 #a4e-b2 the hardest varieties being ,no4 #b of the scale of hardness4 ,sp4 gr41 #b4c4 ,lustre1 pearly1 vitreous1 or dull4 ,color and streak usually white or gray4 ,the principal varieties of gypsum are 7a7 selenite1 which includes all distinctly crystallized or transparent gypsum2 7b7 fibrous gypsum or satin-spar2 7c7 alabaster1 fine-grained1 light-colored1 and translucent4 ,gypsum is easily distinguished from all common minerals resembling it by its softness and the fact that it is not affected by acids4 ,specimen #ag4 #h4 ,calcite4--,carbonate of calcium3 carbon dioxide 7,,co71 #dd2 lime 7,ca,o71 #ef2 #ajj4 ,hexagonal system1 usually in rhombohedrons1 scalenohedrons1 or hexagonal prisms4 ,cleavage rhombohedral and highly perfect 7specimen #ah74 ,also fibrous and compact to coarsely granular1 in stalactitic1 concretionary1 and other forms4 ,hardness1 #b4e-c4e1 usually #c 7see scale of hardness74 ,sp4 gr41 #b4e-b4ge4 ,lustre1 vitreous4 ,color and streak usually white4 ,transparent crystallized calcite is known as ,iceland-spar1 and is remarkable for its strong double refraction4 ,when finely fibrous it makes a satin-spar similar to gypsum4 ,geologically speaking1 calcite is a mineral of the first importance1 being the sole essential constituent of all limestones4 ,it is readily distinguished from allied species by its perfect rhombohedral cleavage2 by its softness1 being easily scratched with a knife2 and above all by its lively effervescence with acids1 for it is the only common mineral effervescing freely with cold dilute acid4 ,to apply this test it is only necessary to touch the specimen with a drop of dilute chlorohydric acid4 ,the effervescence1 of course1 is due to the escape of the carbon dioxide in a gaseous form4 ,specimen #ah4 #i4 ,dolomite4--,carbonate of calcium and magnesium3 carbonate of calcium 7,ca,c,o71 #ed4ce2 carbonate of magnesium 7,mg,c,o71 #de4fe2 #ajj4 ,hexagonal system1 being nearly isomorphous with calcite4 ,rhombohedral cleavage perfect4 ,hardness1 #c4e-d2 sp4 gr41 #b4h-b4i1 being harder and heavier than calcite4 ,lustre1 color1 and streak same as for calcite1 from which it is most easily distinguished by its non-effervescence or only feeble effervescence with cold dilute acid1 though effervescing freely with strong or hot acid4 ,spec4 #ai4 #aj4 ,siderite4--,carbonate of iron3 carbon dioxide 7,,co71 #cg4i2 protoxide of iron 7,fe,o71 #fb4a2 #ajj4 ,crystallization and cleavage essentially the same as for calcite and dolomite4 ,hardness1 #c4e-d4e1 and sp4 gr41 #c4g-c4i4 ,lustre1 vitreous4 ,color1 white1 gray1 and brown4 ,streak1 white4 ,with acid1 siderite behaves like dolomite4 ,it is distinguished from both calcite and dolomite by its high specific gravity1 which is easily explained by the fact that it is largely composed of the heavy element1 iron4 ,with one exception1 the fifteen minerals which we have yet to study belong to the class of silicates1 which includes more than one-fourth of the known species of minerals1 and1 omitting quartz and calcite1 all of the really important rock-constituents4 ,the silicate minerals may be very conveniently divided into two great groups1 the basic and acidic4 ,this is not a sharp division2 on the contrary1 there is a perfectly gradual passage from one group to the other2 and yet this is1 for geological purposes at least1 a very natural classification4 ,the dividing line falls in the neighborhood of #fj per cent4 of silica2 i4e41 all species containing this proportion of silica or less are classed as basic1 since in them the basic elements predominate2 while those containing more than #fj per cent4 of silica are classed as acidic1 because their characteristics are determined chiefly by the acid element or silica4 ,the principal bases occurring in the silicates1 named in the order of their relative importance1 are aluminum1 magnesium1 calcium1 iron1 sodium1 and potassium2 and of these1 magnesium1 calcium1 iron1 and usually sodium1 are especially characteristic of basic species4 ,iron is the heaviest base2 but all the bases1 except sodium and potassium1 are heavier than the acid--silica2 consequently basic minerals must be1 as a rule1 heavier than acidic minerals4 ,and since basic minerals contain more iron than acidic1 they must be darker colored4 ,in general1 we say1 dark1 heavy silicates are basic1 and vice versa4 ,all this is of especial importance because in the rocks nature keeps these two classes separate in a great degree4 #aa4 ,amphibole4--,silicate of aluminum1 magnesium1 calcium1 iron1 and sodium4 ,the bases occur in very various proportions1 forming many varieties2 but the only variety of especial geological interest is hornblende1 the average percentage composition of which is as follows3 silica 7,si,o71 #ej2 alumina 7,al,o71 #aj2 magnesia 7,mg,o71 #ah2 lime 7,ca,o71 #ab2 iron oxide 7,fe,o and ,fe,o71 #h2 and soda 7,na,o71 #b2 #ajj4 ,monoclinic system3 usually in rhombic or six-sided prisms which may be short and thick1 but are more often acicular or bladed4 ,hardness1 #e-f2 sp4 gr41 #b4i-c4d4 ,lustre1 vitreous2 color1 black and greenish black2 and streak similar to color1 but much paler4 ,compare with quartz1 and observe the strong contrast in color possible with minerals having the same lustre4 ,specimen #bj4 #ab4 ,pyroxene4--,like amphibole1 this species embraces many varieties1 and these exhibit a wide range in composition2 but of these augite alone is an important rock-constituent4 ,hence in lithology we practically substitute for amphibole and pyroxene1 hornblende1 and augite respectively4 ,augite is very similar in composition to hornblende1 but contains usually more lime and less alumina and alkali4 ,physically1 too1 these minerals are almost identical1 crystallizing in the same system and in very similar forms1 and agreeing in hardness1 color1 lustre1 and streak4 ,augite is heavier than hornblende1 sp4 gr41 #c4b-c4e4 ,a certain prismatic angle1 which in augite is #hge1 is #abdcj in hornblende4 ,slender1 bladed crystals are more common with hornblende than augite4 ,when examined in thin sections with the polarizer1 augite does not afford the phenomenon of dichroism1 which is strongly marked in hornblende4 ,however1 as these minerals commonly occur in the rocks1 in small and imperfect crystals1 these distinctions can only be observed in thin sections under the microscope2 so that1 as regards the naked eye1 they are practically indistinguishable4 ,it might appear at first that the distinction of minerals so nearly identical is not an important matter2 but nature has decreed otherwise4 ,augite and hornblende are typical examples of basic minerals2 but augite is1 both in its composition and associations1 the more basic of the two4 ,in proof of this we need only to know that it very rarely occurs in the same rock with quartz1 while hornblende is found very commonly in that association4 ,quartz in a rock means an excess of acid or silica1 and almost necessarily implies the absence of highly basic minerals4 ,in other words1 hornblende is often1 and augite very rarely1 found in connection with acidic minerals2 and it is this difference of association chiefly that makes their distinction essential to the proper recognition of rocks2 while at the same time it affords an easy1 though of course not absolutely certain1 means of determining whether the black constituent of any particular rock is hornblende or augite4 ,mica ,family4--,mica is not the name of a single mineral1 but of a whole family of minerals1 including some half-dozen species4 ,only two1 however1--muscovite and biotite1--are sufficiently abundant to engage our attention4 ,these are complex1 basic silicates of aluminum1 magnesium1 iron1 potassium1 and sodium4 ,the crystallization of biotite is hexagonal1 and of muscovite monoclinic2 but both occur commonly in flat six-sided forms4 ,undoubtedly the most important and striking characteristic of the whole mica family is the remarkably perfect cleavage parallel with the basal planes of the crystals1 and the wonderful thinness1 and above all the elasticity1 of the cleavage lamell4 ,the cleavage contrasts the micas with all other common minerals1 and makes their certain identification one of the easiest things in lithology4 ,the micas are soft minerals1 the hardness ranging from #b to #c1 and being usually easily scratched with the nail4 ,sp4 gr4 varies from #b4g-c4a4 ,lustre1 pearly2 and streak1 white or uncolored4 ,the distinguishing features of muscovite and biotite are as follows3-- #ac4 ,muscovite4--,contains #dg per cent4 of silica1 #c per cent4 of sesquioxide of iron1 and #aj per cent4 of alkalies1 chiefly potash2 and the characteristic colors are white1 gray1 and1 more rarely1 brown and yellow4 ,non-dichroic4 ,usually found in association with acidic minerals4 ,the mica used in the arts is muscovite4 ,specimen #ba4 #ad4 ,biotite4--,contains only #cf per cent4 of silica1 #bj per cent4 of oxide of iron1 and #ag per cent4 of magnesia2 colors1 deep black to green4 ,strongly dichroic4 ,commonly occurs with other basic minerals4 ,compare color with per cent4 of iron4 ,these differences are tabulated below3-- ,muscovite ,biotite ,acidic mica4 ,basic mica4 ,non-ferruginous mica4 ,ferruginous mica4 ,potash mica4 ,magnesian mica4 ,white mica4 ,black mica4 ,non-dichroic mica4 ,dichroic mica4 ,feldspar ,family4--,like mica1 feldspar is the name of a family of minerals2 and these are1 geologically1 the most important of all minerals4 ,they are1 above all others1 the minerals of which rocks are made1 and their abundance is well expressed in the name1--feldspar being simply the ,german for field-spar1 implying that it is the common spar or mineral of the fields4 ,chemically1 the feldspars are silicates of aluminum and potassium1 sodium or calcium4 ,they crystallize in the monoclinic and triclinic systems2 and all possess easy cleavage in two directions at right angles to each other1 or nearly so4 ,the general physical characters1 including the cleavage1 are well exhibited in the common species1 orthoclase 7specimen #bb74 ,in hardness the feldspars range from #e to #g1 being usually near #f1 and almost always distinctly softer than quartz4 ,sp4 gr4 varies from #b4e-b4ge2 lustre1 from vitreous to pearly2 color1 from white and gray to red1 brown1 green1 etc41 but usually light4 ,streak1 always white2 rarely transparent4 ,by exposure to the weather1 feldspars gradually lose their alkalies and lime1 become hydrated1 and are changed to kaolin or common clay4 ,a similar change takes place with the micas1 augite1 and hornblende2 but these species1 being usually rich in iron1 make clays which are much darker colored than those derived from feldspars4 ,the fact that the feldspars contain little or no iron undoubtedly explains their low specific gravity and light colors1 as compared with the other minerals just named4 ,the only common minerals for which the feldspars are liable to be mistaken are quartz and the carbonates4 ,from the latter they are easily distinguished by their superior hardness and non-effervescence with acids2 and from the former1 by possessing distinct cleavage1 by being rarely transparent1 by being somewhat softer1 and by changing to clay on exposure to the weather4 ,the feldspars of greatest geological interest are five in number1 and may be classified chemically as follows3-- ,orthoclase1--silicate of aluminum and potassium1 or potash feldspar4 ,albite1--silicate of aluminum and sodium1 or soda feldspar4 ,anorthite1--silicate of aluminum and calcium1 or lime feldspar4 ,oligoclase1--silicate of aluminum and sodium1 and calcium1 or soda-lime feldspar4 ,labradorite1--silicate of aluminum and calcium1 and sodium1 or lime-soda feldspar4 ,this appears like a complex arrangement1 but it can be simplified4 ,orthoclase crystallizes in the monoclinic system1 and all the other feldspars in the triclinic system4 ,with the exception of albite1 which is a comparatively rare species1 the triclinic feldspars all contain less silica than orthoclase2 i4e41 are more basic4 ,this is shown by the subjoined table giving the average composition of each of the feldspars3-- ,si,o ,al,o ,k,o ,na,o ,ca,o ,total4 ,orthoclase1 #fe #ah #ag -- -- #ajj ,albite1 #fh #bj -- #ab -- #ajj ,oligoclase1 #fb #bd -- #i #e #ajj ,labradorite1 #ec #cj -- #d #ac #ajj ,anorthite1 #dc #cg -- -- #bj #ajj ,as we should naturally expect1 the triclinic feldspars occur usually with other basic minerals1 while the monoclinic species1 orthoclase1 is acidic in its associations2 furthermore1 the triclinic feldspars are often intimately associated with each other1 but are rarely important constituents of rocks containing much orthoclase4 ,in other words1 the distinction of orthoclase from the basic or triclinic feldspars is important and comparatively easy1 while the distinction of the different basic feldspars from each other is both unimportant and difficult4 ,hence1 in lithology1 we find it best to put all these basic feldspars together1 as if they were one species1 under the name plagioclase1 which refers to the oblique cleavage of all these feldspars1 and contrasts with orthoclase1 which refers to the right-angled cleavage of that species4 ,this statement of the relations of the feldspars is1 of course1 beyond the comprehension of many children1 and yet it should be understood by the teacher who would lead the children to any but the most superficial views4 #ae4 ,orthoclase4--,this is the common feldspar1 and the most abundant of all minerals1 being the principal constituent of granite1 gneiss1 and many other important rocks4 ,the most characteristic colors are white1 gray1 pinkish1 and flesh-red4 ,specimen #bb4 #af4 ,plagioclase4--,like orthoclase1 these species may be of almost any color2 yet these two great divisions of the feldspars are usually contrasted in this respect4 ,thus1 bluish and grayish colors are most common with plagioclase1 and white or reddish colors with orthoclase4 ,specimen #bc is labradorite1 and1 in every respect1 a typical example of plagioclase4 ,on certain faces and cleavage-surfaces of the plagioclase crystals we may often observe a series of straight parallel lines or bands which are often very fine1--fifty to a hundred in a single crystal4 ,these stri are due to the mode of twinning1 and are of especial importance1 since1 while they are very characteristic of plagioclase1 they never occur in orthoclase4 ,as stated1 these twinning stri in plagioclase are often visible to the naked eye2 and when they are not1 they may usually be revealed by examining a thin section under the microscope with polarized light4 ,plagioclase decays much more rapidly when exposed to the weather than orthoclase4 ,this point becomes perfectly clear when we compare weathered ledges of diabase 7or any trap-rock1 see specimen #b7 and granite2 for plagioclase is the principal constituent of the former rock1 and orthoclase of the latter4 ,hydrous ,silicates4--,many silicates contain water1 and some of these are of great geological importance4 ,what has been stated on a preceding page concerning the softness and lightness of hydrated minerals is especially applicable here2 for all the geologically important hydrous silicates are distinctly softer and lighter than anhydrous minerals of otherwise similar composition4 ,furthermore1 they usually have an unctuous or slippery feel2 and1 with one exception 7kaolin71 are of a green or greenish color4 #ag4 ,kaolinite 7,kaolin74--,hydrous silicate of aluminum3 silica 7,si,o71 #df2 alumina 7,al,o71 #dj2 and water 7,h,o71 #ad2 #ajj4 ,orthorhombic system1 in rhombic or hexagonal scales or plates1 but usually earthy or clay-like4 ,hardness1 #a-b4e2 sp4 gr41 #b4d-b4fe4 ,the pure mineral is white2 but it is usually colored by impurities1 the principal of which are iron oxides and carbonaceous matter4 ,kaolin is the most abundant of all the hydrous silicates1 and it is the basis and often the sole constituent of common clay1--a very common mineral1 but rarely pure4 ,we have already 7p4 #aa7 noticed the mode of origin of kaolin or clay4 ,it results from the decomposition of various aluminous silicate minerals1 especially the feldspars4 ,under the combined influence of carbon dioxide and moisture1 feldspars give up their potassium1 sodium1 and calcium1 and take on water1 and the result is kaolin4 ,this mineral is believed to be always a decomposition product4 ,perhaps the best1 or at least the most convenient1 test for kaolin is the argillaceous odor1 the odor of moistened clay4 ,specimen #bd4 #ah4 ,talc4--,hydrous silicate of magnesium3 silica 7,si,o71 #fc 7acidic72 magnesia 7,mg,o71 #cb2 water 7,h,o71 #e2 #ajj4 ,orthorhombic system1 but rarely in distinct crystals4 ,cleavage in one direction very perfect2 the cleavage lamell are flexible1 but not elastic1 as in mica4 ,hardness1 #a2 see scale4 ,sp4 gr41 #b4ee-b4h4 ,lustre1 pearly4 ,color1 apple-green to white2 and streak1 white4 ,the feel is very smooth and greasy2 and1 in connection with the color and foliation1 affords the best means of distinguishing talc from allied minerals4 ,talc sometimes results from the alteration of augite1 hornblende1 and other minerals1 but it is not always nor usually an alteration product4 #ai4 ,serpentine4--,hydrous silicate of magnesium3 silica 7,si,o71 #dd 7basic72 magnesia 7,mg,o71 #dd2 water 7,h,o71 #ab2 #ajj4 ,essentially amorphous4 ,hardness1 #b4e-d2 sp4 gr41 #b4e-b4fe4 ,lustre1 greasy1 waxy1 or earthy4 ,color1 various shades of green and usually darker than talc1 but streak always white4 ,feel1 smooth1 sometimes greasy4 ,distinguished from talc by its hardness1 compactness1 and darker green4 ,sometimes results from the alteration of olivine and other magnesian minerals1 but usually we are to regard it as an original mineral4 ,specimen #be4 #bj4 ,chlorite4--,this is1 properly1 the name of a group of highly basic minerals of very variable composition1 but they are all essentially hydrous silicates of aluminum1 magnesium1 and iron2 and the average composition of the most abundant species1 prochlorite1 is as follows3 silica 7,si,o71 #cj2 alumina 7,al,o71 #ah2 magnesia 7,mg,o71 #ae2 protoxide of iron 7,fe,o71 #bf2 and water 7,h,o71 #aa2 #ajj4 ,the chlorites crystallize in several different systems1 but in all there is a highly perfect cleavage in one direction1 giving1 as in talc1 a foliated structure with flexible but inelastic lamin4 ,the cleavage scales1 however1 are sometimes minute1 and the structure massive or granular4 ,hardness of prochlorite1 #a-b2 between talc and serpentine4 ,sp4 gr41 #b4gh-b4if4 ,all the chlorites have a pearly to vitreous lustre4 ,color usually some shade of green2 in prochlorite a dark or blackish green1 darker than serpentine1 as that is darker than talc4 ,streak1 a lighter1 whitish green4 ,less unctuous than talc1 but more so than serpentine4 ,the chlorites are produced very commonly1 but not generally1 by the alteration of basic anhydrous silicates1 like augite and hornblende4 ,specimen #bf4 #ba4 ,hydro-mica4--,this1 too1 is properly the name of a group of minerals2 but for geological purposes they may be regarded as one species4 ,taking a general view of the composition1 these are simply the anhydrous or ordinary micas1 which we have already studied1 with from #e to #aj per cent4 of water added4 ,in crystallization and structure they are essentially mica-like4 ,although not distinctly softer than the common micas1 they are lighter1 always more unctuous and slippery1 and usually of a greenish color4 ,the micaceous structure with elastic lamin serves to distinguish the hydro-micas from other hydrous silicates4 #bb4 ,glauconite4--,hydrous silicate of aluminum1 iron1 and potassium3 silica 7,si,o71 #ej2 alumina 7,al,o71 protoxide of iron 7,fe,o71 and potash 7,k,o71 together1 #da2 and water 7,h,o71 #i2 #ajj4 ,amorphous1 forming rounded and generally loose grains1 which often have a microscopic organic nucleus4 ,it is dull and earthy1 like chalk1 and always soft1 green1 and light1 but not particularly unctuous4 ,glauconite is the principal1 often the sole1 constituent of the rock greensand1 which occurs abundantly in the newer geological formations1 and is now forming in the deep water of the ,gulf of ,mexico and along our ,atlantic sea-board4 ,specimen #bg4 ,this completes our list of minerals occurring chiefly as essential constituents of rocks2 and following are three of the more common and important minerals occurring chiefly as accessory1 rarely as essential1 rock-constituents4 #bc4 ,chrysolite 7,olivine74--,silicate of magnesium and iron3 silica 7,si,o71 #da2 magnesia 7,mg,o71 #ea2 protoxide of iron 7,fe,o71 #h2 #ajj4 ,orthorhombic system2 but usually in irregular glassy grains4 ,hardness1 #f-g4 ,sp4 gr41 #c4c-c4e4 ,lustre1 vitreous2 color1 usually some shade of green2 and streak1 white4 ,chrysolite sometimes closely resembles quartz1 but its green color usually suffices to distinguish it4 ,it is a common constituent of basalt and allied rocks4 ,by absorption of water it is changed into serpentine and talc4 ,see examples in specimen4 #bd4 ,garnet4--,the composition of this mineral is extremely variable2 but the most important variety is a basic silicate of aluminum and iron3 silica 7,si,o71 #cg2 alumina 7,al,o71 #bj2 and protoxide of iron 7,fe,o71 #dc2 #ajj4 ,isometric system1 usually in distinct crystals1 twelve-sided 7dodecahedrons7 and twenty-four-sided 7trapezohedrons7 forms being most common4 ,hardness1 #f4e-g4e2 average as hard as quartz4 ,sp4 gr41 #c4ae-d4c2 compare with the high percentage of iron4 ,lustre1 vitreous2 colors1 various1 usually some shade of red or brown2 and streak1 white4 ,some varieties contain iron enough to make them magnetic4 ,garnet is easily distinguished by its form1 color1 and hardness from all other minerals4 ,it is a common but not an abundant mineral1 occurring most frequently in gneiss1 mica schist1 and other stratified crystalline rocks4 ,see examples in specimen4 #be4 ,pyrite4--,sulphide of iron3 sulphur1 #ec4c2 iron1 #df4g2 #ajj4 ,isometric system1 occurring usually in distinct crystals1 the cube and the twelve-sided form known as the pyritohedron being the most common4 ,hardness1 #f-f4e1 striking fire with steel4 ,sp4 gr41 #d4h-e4b2 heavy because rich in iron4 ,lustre1 metallic and splendent4 ,color1 pale1 brass-yellow1 and streak1 greenish or brownish4 ,pyrite is sometimes mistaken for gold1 but it is not malleable2 while its color1 hardness1 and specific gravity1 combined1 easily distinguish it from all common minerals4 ,as an accessory rock-constituent1 pyrite occurs usually in isolated cubes or pyritohedrons4 ,specimen #aj4 ,textures of ,rocks4 ,texture is a general name for those smaller structural features of rocks which can be studied in hand specimens1 and which depend upon the forms and sizes of the constituent particles of the rocks1 and the ways in which these are united4 ,by 8constituent particles0 we mean1 not the atoms or molecules of matter composing the rocks1 but the pebbles in conglomerate1 grains of sand in sandstone1 crystals of quartz1 feldspar1 and mica in granite1 etc4 ,the four most important textures are3-- 7#a7 ,fragmental texture4--,the rock is composed of mere irregular1 angular1 or rounded1 but visible1 fragments4 ,examples3 sand1 sandstone1 gravel1 conglomerate1 etc4 ,specimens #cj1 #ca1 #bh1 #bi4 7#b7 ,crystalline texture4--,the constituent particles are chiefly1 at least1 distinctly crystalline1 as shown either by external form1 or cleavage1 or both4 ,examples3 granite1 diabase1 gneiss1 etc4 ,specimens #de1 #a1 #da4 7#c7 ,compact texture4--,the constituent particles are indistinguishable by the naked eye1 but become visible under the microscope1 appearing as separate crystalline grains or as irregular fragments4 ,in other words1 if1 in the case of either the granular or crystalline textures1 we conceive the particles to become microscopically small1 then we have the compact texture4 ,examples3 clay1 slate1 many limestones1 basalt1 etc4 ,specimens #cd1 #ce1 #ci4 7#d7 ,vitreous texture4--,the texture of glass1 in which the constituent particles are absolutely invisible even with the highest powers of the microscope1 and may be nothing more than the molecules of the substance1 which thus1 so far as our powers of observation are concerned1 presents a perfectly continuous surface4 ,examples3 obsidian1 glassy quartz1 and some kinds of coal4 ,specimens #dg1 #ae4 ,these four textures1 which1 it will be observed1 are determined by the forms and sizes of the constituent particles1 may be called the primary textures1 because every rock must possess one of them4 ,we cannot conceive of a rock which is neither fragmental1 crystalline1 compact1 nor vitreous4 ,but in addition to one of the primary textures1 a rock may or may not have one or more of what may be called secondary textures4 ,these are determined by the way in which the particles are united1 the mode or pattern of the arrangement1 etc4 ,following are definitions of the principal secondary textures3-- 7#a7 ,laminated texture4--,this exists where the particles are arranged in thin1 parallel layers1 which may be marked simply by planes of division1 or the alternate layers may be composed of particles differing in composition1 form1 size1 or color1 etc4 ,among the laminated textures we thus distinguish3 7a7 the banded texture1 where the layers are contrasted in color1 texture1 or composition1 but cohere1 so that there is no cleavage or easy splitting parallel with the stratification2 and 7b7 the schistose or shaly texture1 where such fissility or stratification-cleavage exists4 ,if a fragmental1 compact1 or vitreous rock is fissile1 we use the term shaly2 but a fissile1 crystalline rock is described as schistose4 ,the banded texture may occur with the fragmental1--banded sandstones1 etc42 with the crystalline1--many gneisses1 etc4 7specimen #da72 with the compact1--many slates1 limestones1 felsites1 etc4 7specimens #cd1 #db72 with the vitreous1--banded obsidian1 furnace slags1 and some coal4 ,the schistose texture may occur with the crystalline1--mica schist1 etc4 7specimen #dc72 and the shaly texture with the compact and fragmental1 but rarely with the vitreous4 7#b7 ,porphyritic texture4--,we have this texture when separate and distinct crystals of any mineral1 but most commonly of feldspar1 are enclosed in a relatively fine-grained base or matrix1 which may be either crystalline1 compact1 or vitreous1 but rarely fragmental4 ,specimens #e1 #f1 #g are examples of the porphyritic compact texture4 7#c7 ,concretionary texture4--,when one or more constituents of a rock have the form1 in whole or in part1 not of distinct angular crystals1 but of rounded concretions1 the texture is described as concretionary1 the concretions taking the place in this texture of the isolated crystals in the porphyritic texture4 ,this texture occurs in connection with all the primary textures1 but the most familiar example is olitic limestone4 7#d7 ,vesicular texture4--,a rock has this texture when it contains numerous small cavities or vesicles4 ,these are most commonly produced by the expansion of steam and other vapors when the rock is in a plastic state2 and hence the vesicular texture is found chiefly in volcanic rocks4 ,except rarely1 it is associated only with the compact texture1--ordinary stony lavas 7specimen #di72 and with the vitreous texture1--pumice 7specimen #dh74 7#e7 ,amygdaloidal texture4--,in the course of time the vesicles of common lava are often filled with various minerals deposited by infiltrating waters1 giving rise to the amygdaloidal texture1 from the ,latin amygdalum1 an almond1 in allusion to a common form of the vesicles1 or amygdules1 as they are called1 after being filled4 ,the amygdaloidal texture is thus necessarily preceded by the vesicular1 and is limited to the same classes of rocks4 ,specimen #ej4 ,besides the foregoing1 there are many minor secondary textures4 ,the rocks known as tufas have what may be called the tufaceous texture4 ,then we have kinds of texture depending on the strength of the union of the particles1 as strong1 weak1 friable1 earthy1 etc4 ,classification of ,rocks4 ,having finished our preliminary observations on the characteristics of rocks1 we are now about ready to begin a systematic study of the rocks themselves2 but it is needful first to say a few words about the classification of rocks1 since upon this depends not only the order in which we shall take the rocks up1 but also the ideas that will be imparted concerning their relations and affinities4 ,the classifications which have been proposed at different times are almost as numerous as the rocks themselves4 ,some of these are confessedly1 and even designedly1 artificial1 as when we classify stones according to their uses in the arts1 etc4 ,but we want something more scientific1 a natural classification2 that is1 one based upon the natural and permanent characteristics of rocks4 ,rocks have been classified according to chemical composition1 mineralogical composition1 texture1 color1 density1 hardness1 etc42 but these arrangements1 taken singly or all combined1 are inadequate4 ,a natural classification may be defined as a concise and systematic statement of the natural relations existing among the objects classified4 ,now the most important relations existing among rocks are those due to their different origins4 ,we must not forget that lithology is a branch of geology1 and that geology is first of all a dynamical science4 ,the most important question that can be asked about any rock is1 not ,what is it made of8 but ,how was it made8 ,what were the general forces or agencies concerned in its formation8 ,rocks are the material in which the earth's history is written1 and what we want to know first concerning any rock is what it can tell us of the condition of that part of the earth at the time it was made and subsequently4 ,classification of ,rocks4 ---------------------------------------- ---------------------------------- ,sedimentary or ,stratified ,rocks4 ---------------------------------------- ---------------------------------- ,,mechanically ,,formed4 ---------------------------------------- -------------------------------- ,unconsolidated4 ,consolidated4 ---------------------------------------- -------------------------------- ,conglomerate ,gravel4 ,conglomerate4 group4 ---------------------------------------- -------------------------------- ,arenaceous ,sand4 ,sandstone4 group4 ---------------------------------------- -------------------------------- ,argillaceous ,clay4 ,slate4 group4 ---------------------------------------- -------------------------------- ,,chemically ,,and ,,organically ,,formed4 ---------------------------------------- -------------------------------- ,coal ,iron-ore ,calcareous ,metamorphic group 7,silicates74 group4 group4 group4 ,acidic4 ,basic4 _/----_/---- _/-------_/------- #he #hj #gj #fj #ej #dj #cj ---------------------------------------- ------------------------------- ,feldspathic4 ---------------------------------------- ------------------------------- ,peat4 ,limonite4 ,limestone4 ,gneiss4 ,diorite4 3 ,lignite4 ,hematite4 ,dolomite4 444444444444444444444444444 3 ,bit4 ,coal4 ,magnetite4 ,gypsum4 3 3 3 3 3 ,anthracite4 ,siderite4 ,rock-salt4 3 3,syenite4 ,norite4 3 ,graphite4 ,phosphate 3 3 44444444444444 3 ,asphaltum4 ,rock4 3 3 3 3 3 ---------------------------------------- ------------------------------- ,non-,feldspathic4 ---------------------------------------- ------------------------------- ,siliceous 3 ,mica ,schist4 3 3 group4 444444444444444444444444444 3 ----------- 3 3 3 3 3 ,tripolite4 3,hornbl4 ,schist4 ,amphibolite4 ,flint4 344444444444444 444444443 ,siliceous 3 3 3 3 3 ,tufa4 3 ,talc ,schist4 3,chl4 ,schist4 ,novaculite4 3 344444444 3 4444444444 3 3 3 3 3 3 3 ,greensand4 ,serpentine4 3 3 44444444444 44444444 3 3 3 3 3 ---------------------------------------- ------------------------------- ,eruptive or ,unstratified ,rocks4 ---------------------------------------- --------------------------------- ,,plutonic4 ,feldspathic4 ------------------------------------- 3 ,granite4 3 ,diorite4 3 444444444444444444 44444444 3 3 3 3 3 3 3 3 ,syenite4 ,diabase4 3 3 4444444 44444443 3 3 3 3 3 ------------------------------------- ,,volcanic4 ,this part of the classification ,feldspathic4 is a blank1 for the reason that ------------------------------------- no eruptive rocks are known 3,rhyolite4 3 ,andesite4 3 which are chiefly composed of 444444444444444444444444444 3 minerals belonging to the classes 3 3 3 3 3 of ,native ,elements1 ,chlorides1 3 3 ,trachyte4 ,basalt4 ,oxides1 ,sulphates1 or ,carbonates2 4444444444444443 i4e41 all eruptive rocks1 so 3 3 3 3 3 far as known1 are principally 3,obsidian4 3 ,tachylite43 composed of minerals belonging 4444444444444444444444444444 3 to the class of ,silicates4 3 3 3 3 3 ,petrosilex4 3,porphyrite43 4444444444444444444444444444 3 3 3 3 3 3 3 3 ,felsite4 ,melaphyr4 3 3 4444444 44444443 3 3 3 3 3 ---------------------------------------- --------------------------------- ,the geological agencies1 as we have already learned1 may be arranged in two great classes3 first1 the aqueous or superficial agencies originating in the solar heat1 and producing the sedimentary or stratified rocks2 and1 second1 the igneous or subterranean agencies originating in the central or interior heat1 and producing the eruptive or unstratified rocks4 ,hence1 we want to know first of any rock whether it is of aqueous or igneous origin4 ,then1 if it is a sedimentary rock1 whether it has been formed by the action chiefly of mechanical forces1 or of chemical and organic forces4 ,and1 if it is an eruptive rock1 whether it has cooled and solidified below the earth's surface in a fissure1 and is a dike or trappean rock1 or has flowed out on the surface and cooled in contact with the air1 and thus become an ordinary lava or volcanic rock4 ,here we have the outlines of our classification1 and it will be observed that we have simply reached the conclusion1 in a somewhat roundabout manner1 that there should always be a general correspondence between the classification of rocks and the classification of the forces that produce them4 ,the general plan of the preceding scheme of the classification must now be clear1 and the details will be explained as we go along4 ,descriptions of ,rocks4 #a4--,sedimentary or ,stratified ,rocks4 #a4 ,,mechanically ,,formed ,,or ,,fragmental ,,rocks4--,these consist of materials deposited from suspension in water1 and the process of their formation is throughout chiefly mechanical4 ,the materials deposited are mere fragments of older rocks2 and1 if the fragments are large1 we call the newly deposited sediment gravel2 if finer1 sand2 and1 if impalpably fine1 clay4 ,these fragmental rocks cannot be classified chemically1 since the same handful of gravel1 for instance1 may contain pebbles of many different kinds of rocks1 and thus be of almost any and very variable composition4 ,such chemical distinctions as can be established are only partial1 and the classification1 like the origin1 must be mechanical4 ,accordingly1 as just shown1 we recognize three principal groups based upon the size of the fragments2 viz43-- 7#a7 ,conglomerate group4 7#b7 ,arenaceous group4 7#c7 ,argillaceous group4 ,this mode of division is possible and natural1 simply because1 as we observed in an early experiment1 materials arranged by the mechanical action of water are always assorted according to size4 ,when first deposited1 the gravel1 sand1 and clay are1 of course1 perfectly loose and unconsolidated2 but in the course of time they may1 under the influence of pressure1 heat1 and chemical action1 attain almost any degree of consolidation1 becoming conglomerate1 sandstone1 and slate1 respectively4 ,the pressure may be vertical where it is due to the weight of newer deposits1 or horizontal where it results from the cooling and shrinking of the earth's interior4 ,the heat may result from mechanical movements1 or contact with eruptive rocks2 or it may be due simply to the burial of the sediments1 which1 it will be seen1 must virtually bring them nearer the great source of heat in the earth's interior1 on the same principle that the temperature of a man's coat1 on a cold day1 is raised by putting on an overcoat4 ,the effect of the heat1 ordinarily1 is simply drying1 coperating with the pressure to expel the water from the sediments2 but1 if the temperature is high1 it may bake or vitrify them1 just as in brick-making4 ,sediments are consolidated by chemical action when mineral substances1 especially calcium carbonate1 the iron oxides1 and silica are deposited between the particles by infiltrating waters1 cementing the particles together4 ,this principle is easily demonstrated experimentally by taking some loose sand and wetting it repeatedly with a saturated solution of some soluble mineral1 like salt or alum1 allowing the water to evaporate each time before making a fresh application4 ,the interstices between the grains are gradually filled up1 and the sand soon becomes a firm rock4 ,but the student should clearly understand that1 in geology1 gravel1 sand1 and clay are just as truly rocks before their consolidation as after4 ,it is plain then that in each of the principal groups of fragmental rocks we must recognize an unconsolidated division and a consolidated division4 7#a7 ,conglomerate group4--,the rocks belonging in this group we know before consolidation as gravel1 and after consolidation as conglomerate4 ,gravel4--,the pebbles1 as we have already seen1 are usually1 though not always1 well rounded or water-worn2 and they may be of any size from coarse grains of sand to boulders4 ,as a rule1 however1 the larger pebbles1 especially1 are of approximately uniform size in the same bed or layer of gravel1 with1 of course1 sufficient fine material to fill the interstices4 ,although the same limited mass of gravel may show the widest possible range in chemical and mineralogical composition1 yet hard rocks are evidently more likely than soft rocks to form pebbles2 and hence quartz and quartz-bearing rocks usually predominate in gravels4 ,specimen #bh4 ,conglomerate4--,consolidated gravel4 ,children should be led to an appreciation of this point by a careful comparison of the forms of the pebbles in the gravel and conglomerate4 ,the conglomerate seems to contain a larger proportion of fine material than ordinary gravel4 ,but this is because the gravel is usually1 as with our specimen1 taken from the surface of the beach1 where1 of course1 the pebbles are clean and separate2 but if it had remained there to be covered by a subsequently deposited layer1 enough fine stuff would have been sifted into the holes to fill them4 ,and in the finished gravel1 just as in the conglomerate1 the pebbles are usually closely packed1 with just sufficient sand and clay1 or paste1 as the material in which the pebbles are imbedded is called1 to fill the interstices4 ,the paste is usually similar in composition to the pebbles1 with this difference3 hard materials predominate in the pebbles and soft in the paste4 ,stratified rocks generally show the stratification in parallel lines or bands differing in color1 composition1 etc42 but nothing like this can be detected in our specimens of conglomerate2 and the question might be asked1 ,how do we know that this is a stratified rock8 ,in answer1 it can be said that our hand-specimens appear unstratified simply because the rock is so coarse2 but when we look at large masses1 and especially when we see it in place in the quarry1 that parallel arrangement of the material which we call stratification is usually very evident2 and we often see precisely the same thing in gravel banks4 ,it is1 however1 wholly unnecessary that we should see the stratification in order to know certainly that this is a stratified or aqueous rock1 because the forms of the pebbles show very plainly that they have been fashioned and deposited by moving water2 and we have in the smallest specimen proof positive that our conglomerate is a consolidated sea-beach4 ,conglomerate shows the same variations in composition and texture as gravel2 it may be composed of almost any kind of material in pebbles of almost any size4 ,we recognize two principal varieties of conglomerate based on the forms of the pebbles2 if1 as is usual1 these are well rounded and water-worn1 the rock is true pudding-stone 7specimen #bi72 but1 if they are angular1 or show but little wear1 it is called breccia4 7#b7 ,arenaceous ,group4--,the conglomerate group passes insensibly into the arenaceous group2 for1 from the coarsest gravel to the finest sand1 the gradation is unbroken1 and every sandstone is merely a conglomerate on a small scale4 ,sand4--,like gravel1 sand may be of almost any composition1 but as a rule it is quartzose2 quartz1 on account of its hardness and the absence of cleavage1 being better adapted than any other common mineral to form sand4 ,where the composition of a sand is not specified1 a quartzose sand is always understood4 ,by examining a typical sand with a lens1 and noting the glassy appearance of the grains1 and then testing their hardness on a piece of glass1 which they will scratch as easily as quartz1 the pupil is readily convinced that each grain is simply an angular fragment of quartz4 ,specimen #cj4 ,sandstone4--,consolidated sand4 ,in proving this1 children will notice first the granular or sandy appearance of the sandstone2 and then1 with the lens1 that the grains in the sandstone have the same forms as the sand-grains4 ,the stratification cannot be seen very distinctly in our hand-specimens1 but in larger masses it is usually very plain1 as may be observed in the blocks used for building1 and still better in the quarries4 ,however1 even if the stratification were not visible to the eye1 we could have no doubt that sandstone is a mechanically formed stratified rock2 because the form of the grains1 just as in the conglomerate1 tells us that4 ,many sandstones1 too1 contain the fossil remains of plants and animals1 and these are always regarded as affording positive proof that the rocks containing them belong to the aqueous or stratified series4 ,there are many varieties of sandstone depending upon differences in composition1 texture1 etc41 but we have not space to notice them in detail4 ,in sandstone1 just as in sand1 quartz is the predominant constituent1 although we sometimes find varieties composed largely or entirely of feldspar1 mica1 calcite1 or other minerals4 ,specimen #ca is an example of the architectural variety known as freestone1 which is merely a fine-grained1 light-colored1 uniform sandstone1 not very hard1 and breaking with about equal freedom in all directions4 ,the consolidation of sandstones is due chiefly to chemical action4 ,the cementing materials are commonly either3 ferruginous 7iron oxides71 giving red or brown sandstones2 calcareous1 forming soft sandstones1 which effervesce with acid if the cement is abundant2 or siliceous1 making very strong1 light-colored sandstones4 ,ferruginous sandstones are the most valuable for architectural purposes2 for1 while not excessively hard1 they have a very durable cement4 ,siliceous sandstones are too hard2 and the calcareous varieties crumble when exposed to the weather because the cement is soluble in water containing carbon dioxide1 as all rain-water does4 ,specimen #cb is a good example of a ferruginous sandstone1 and it is coarse enough so that we can see that each grain of quartz is coated with the red oxide of iron4 ,the mica scales visible here and there in this specimen are interesting as showing that the grains are not necessarily all quartz2 and it is important to observe that the mica was not made in the sandstone1 but1 like the quartz1 has come from some older rock4 ,quartzite4--,this rock is simply an unusually hard sandstone4 ,now the hardness of any rock depends upon two things3 7#a7 the hardness of the individual grains or particles2 and 7#b7 the firmness with which they are united one to another4 ,therefore1 the hardest sandstones must be those in which grains of quartz are combined with an abundant siliceous cement2 and that is precisely what we have in a typical quartzite1 such as specimen #cc4 ,quartzite is distinguished1 in the hand-specimen1 from ordinary quartz by its granular texture 7compare specimens #ae and #cc72 and of course in large masses the stratification is an important distinguishing feature4 #c4 ,argillaceous group4--,just as the conglomerate group shades off gradually into the arenaceous group1 so we find it difficult to draw any sharp line of division between the arenaceous group and the argillaceous1 but we pass from the largest pebble to the most minute clay-particle by an insensible gradation4 ,for the sake of convenience1 however1 we draw the line at the limit of visibility1 and say that in the true clay and slate the individual particles are invisible to the naked eye2 in other words1 these rocks have a perfectly compact texture1 while the two preceding groups are characterized by a granular texture4 ,although clay1 like sand and gravel1 may be of almost any composition1 yet it usually consists chiefly1 often entirely1 of the mineral kaolin4 ,the reason for this is easily found4 ,quartz resists both mechanical and chemical forces1 and is rarely reduced to an impalpable fineness2 but all the other common minerals1 such as feldspar1 hornblende1 mica1 and calcite1 on account of their cleavage and inferior hardness1 are easily pulverized2 but it is practically impossible that this should happen without their being broken up chemically at the same time4 ,decomposition follows disintegration2 and1 while calcite is completely dissolved and carried away1 the other minerals are reduced1 as we have seen1 to impalpable hydrous silicates of aluminum1 i4e41 to kaolin4 ,hence1 we find that the fragmental rocks are composed principally of two minerals1 quartz and kaolin1--the former predominating in the conglomerate and arenaceous groups1 and the latter in the argillaceous group4 ,clay4--,that kaolin is the basis of common clay is proved by the argillaceous odor1 which is so characteristic of moist clay4 ,pure kaolin clay is white and impalpable1 like ,china clay2 but pure clays are the exception4 ,they often become coarse and gritty by admixture with sand1 forming loam2 and they also usually contain more or less carbonaceous matter1 which makes black clays2 or more or less ferrous oxide1 which makes blue clays2 or more or less ferric oxide1 which makes red1 brown1 and yellow clays4 ,by mixing these coloring materials in various proportions1 almost any tint may be explained4 ,clays are sometimes calcareous1 from the presence of shells and shell-fragments or of pulverized limestone4 ,these usually effervesce with acid1 and are commonly known as marl4 ,it is the calcareous material in a pulverulent and easily soluble condition that makes the marls valuable as soils4 ,slate4--,consolidated clay4 ,the compact texture and argillaceous odor are usually sufficient to prove this4 ,to get the odor we need simply to breathe upon the specimen1 and then smell of it4 ,we find all degrees of induration in clay4 ,it sometimes1 as every one knows1 becomes very hard by simple drying2 but this is not slate1 and no amount of mere drying will change clay into slate2 for1 when moistened with water1 the dried clay is easily brought back to the plastic state4 ,to make a good slate1 the induration must be the result of pressure1 aided probably to some extent by heat4 ,true slate1 then1 is a permanently indurated clay which will not soak up and become soft when wet4 ,slate is usually easily scratched with a knife1 and it is distinguished from limestone by its non-effervescence with acid4 ,as we should expect1 it shows precisely the same varieties in color and composition as clay4 ,a good assortment of colors is afforded by the roofing-slates4 ,specimen #cd is a typical slate1 for it not only has a compact texture and argillaceous odor1 but it is very distinctly stratified4 ,the stratification is marked by alternating bands of slightly different colors1 and is much finer and more regular than we usually observe in sandstone1 and of course entirely unlike the stratification of conglomerate4 ,these differences are characteristic4 ,some slates1 however1 are so homogeneous that the stratification is scarcely visible in small pieces4 ,thus the roofing-slates 7specimen #ce7 rarely show the stratification2 for it is an important fact that the thin layers into which this variety splits are entirely independent of the stratification4 ,this is the structure known as slaty cleavage2 it is not due to the stratification1 but is developed in the slate subsequently to its deposition1 by pressure4 ,some roofing-slates1 known as ribbon-slates1 show bands of color across the flat surfaces4 ,these bands are the true bedding1 and indicate the absolute want of conformity between this structure and the cleavage4 ,few rocks are richer in fossils than slate1 and these prove that it is a stratified rock4 ,slate which splits easily into thin layers parallel with the bedding is known as shale4 ,porcelainite4--,this is clay or slate which has been baked or partially vitrified by heat so as to have the hardness and texture of porcelain4 #b4 ,,chemically ,,and ,,organically ,,formed ,,rocks4--,we have already learned that from a geological point of view the differences between chemical and organic deposition are not great1 the process being essentially chemical in each case2 and since the limestones and some other important rocks are deposited in both ways1 it is evidently not only unnatural1 but frequently impossible1 to separate the chemically from the organically formed rocks4 ,unlike the fragmental rocks1 the rocks of this division not only admit1 but require1 a chemical classification4 ,this is natural because they are of chemical origin2 and it is practicable because1 with few exceptions1 only one class of minerals is deposited at the same time in the same place1--a very convenient and important fact4 ,therefore our arrangement will be mineralogical1 thus3-- 7#a7 ,coal ,group4 7#b7 ,iron-ore ,group4 7#c7 ,siliceous ,group4 7#d7 ,calcareous ,group4 7#e7 ,metamorphic ,group 7,silicates74 ,most of the silicate rocks are mixed1 i4e41 are each composed of several minerals2 but some silicate rocks and all the rocks of the other divisions are simple1 each species consisting of a single mineral only4 7#a7 ,coal ,group4--,these are entirely of organic origin1 and include two allied series1 which are always merely the more or less extensively transformed tissues of plants or animals2 viz43-- ,coals and ,bitumens4--,at the first lesson we examined a sample of peat 7specimen #h71 and considered the general conditions of its formation1 peat being in every instance simply partially decayed marsh vegetation4 ,it was also stated that1 as during the lapse of time the transformation becomes more complete1 the peat is changed in succession to lignite1 bituminous coal1 anthracite1 and graphite4 ,the coals1 indeed1 make a very beautiful and perfect series1 whether we consider the composition--there being a gradual1 progressive change from the composition of ordinary woody fibre in the newest peat to the pure carbon in graphite1--or the degree of consolidation and mineralization--since there is a gradual passage from the light1 porous peat1 showing distinctly the vegetable forms1 to the heavy crystalline graphite1 bearing no trace of its vegetable origin4 ,this relation is easily appreciated by a child1 if a proper series of specimens is presented4 ,the coals also make a chronological series1 graphite and anthracite occurring only in the older formations1 and lignite and peat in the newer1 while bituminous coal is found in formations of intermediate age4 ,bituminous coal is the typical1 the representative coal2 and from a good specimen of this variety we may learn two important facts3-- 7#a7 ,that true coals1 no less than peat1 are of vegetable origin4 ,to see this we must look at the flat or charcoal surfaces of the coal4 ,these soil the fingers like charcoal1 and usually show the vegetable forms distinctly4 7#b7 ,that coals are stratified rocks4 ,these dirty charcoal surfaces always coincide with the stratification1 being merely the successive layers of vegetation deposited and pressed together to build up the coal2 and when we look at the edge of the specimen the stratification shows plainly enough4 ,the bitumens form a similar though less perfect series1 beginning with the organic tissues1 and ending1 in the opinion of some of the best chemists and mineralogists1 with diamond4 ,in fact the coals and bitumens form two distinct but parallel series4 ,the coals are exclusively of vegetable origin1 while the bitumens are largely of animal origin4 ,the organic tissues in which the two series originate are chemically similar1--the animal tissues1 which produce the lighter forms of bitumen1 however1 containing more hydrogen and less carbon and oxygen than vegetable tissues2 while the final terms1 as just shown1 are probably chemically identical1 being pure carbon1--graphite for the coals and diamond for the bitumens2 so that the entire process of change in each series is essentially carbonization1 a gradual elimination of the gaseous elements1 oxygen and hydrogen1 until pure solid carbon alone remains4 ,the principal differences between the coals and bitumens are the following3-- ,coals are rich in carbon1 with some oxygen and little hydrogen4 ,bitumens are rich in hydrogen1 with some carbon and little or no oxygen4 ,coals are entirely insoluble4 ,bitumens are soluble in ether1 benzole1 turpentine1 etc41 and the solid forms are soluble in the more fluid1 naphtha-like varieties4 ,coals are never liquid1 and cannot be melted or1 with trifling exceptions1 even softened by heat4 ,many bitumens are naturally liquid1 and all become so on the application of heat4 ,the coals partake of the characteristics of their chief constituent element1 carbon1 the most thoroughly solid substance known2 while the bitumens similarly show the influence of hydrogen1 the most perfectly fluid substance known4 ,the two bitumens of the greatest geological importance are asphaltum or mineral pitch and petroleum2 but these substances are too familiar to require any farther description here4 7#b7 ,iron-ore ,group4--,these interesting and important stratified rocks include the three principal oxides of iron1--limonite1 hematite1 and magnetite1--as well as the carbonate of iron1 siderite2 and the rocks have essentially the same characteristics as the minerals4 ,in economical importance they are second only to the coals2 and the history of their formation through the agency of organic matter is one of the most interesting chapters in chemical geology 7see page #bf74 ,the three oxides are easily distinguished from each other by the colors of their powders or streaks1 and the magnetism of magnetite1 and from all other common rocks by their high specific gravity4 ,magnetite is the richest in iron1 and limonite the poorest4 ,as regards the degree of crystallization and order of occurrence in the formations1 they form a series parallel with the coal series1 thus3-- ,limonite1 never crystalline1 and found in recent formations4 ,hematite1 often crystalline1 and found in older formations4 ,magnetite1 always crystalline1 and found in oldest formations4 ,siderite effervesces with strong acid2 and this separates it from all other rocks1 except limestone and dolomite2 and from these it is distinguished by its high specific gravity4 ,as a mineral1 siderite is often light colored2 but as a rock it is always dark1 and usually black1 from admixture chiefly of carbonaceous matter4 ,in studying dynamical geology1 we have learned 7page #bh7 the reason for the intimate association of siderite with beds of coal1 and this accounts equally for the carbon contained in the rock itself4 ,the connection of this rock with the coal-formations adds much to its value as an ore of iron4 ,finally1 the iron-ores1 at least where of much economical importance1 are truly stratified4 ,this can often be seen in hand-specimens2 and is well shown by their relations to other rocks1 in quarries and mines2 and in many cases1 for limonite and hematite1 by the fossils which they contain4 7#c7 ,siliceous ,group4--,these rocks are composed of pure silica in the forms of quartz and opal4 ,when first deposited1 whether organically1 like tripolite1 or chemically1 like siliceous tufa1 the siliceous rocks are soft and light1 and the silica is in the form of opal4 ,subsequently it changes to quartz1 and the rocks assume the much harder and denser forms of chert and novaculite1 respectively4 ,tripolite or ,diatomaceous ,earth4--,this interesting rock is soft1 light1 and looks like clay2 but it is lighter1 and the argillaceous odor is faint or wanting4 ,it does not effervesce with acid4 ,hence1 it is neither clay nor chalk4 ,notwithstanding its softness1 it is really composed of a hard substance1 viz41 silica1 in the form known as opal4 ,by rubbing off a little of the dust1 and examining it under the microscope1 we easily prove that the silica is mainly or entirely of organic origin2 for the dust is seen to be simply a mass of more or less fragmentary organic remains1 occurring in great variety1 and of wonderful beauty and minuteness4 ,there are few rocks so unpromising on the exterior1 and yet so beautiful within4 ,we have already learned that these organic bodies are principally ,diatom cases1 ,radiolaria shells1 and ,sponge spicules4 ,we can form some idea of their minuteness from ,ehrenberg's estimate that a single cubic inch of pure tripolite contained no less than #da1jjj1jjj1jjj organisms4 ,the lightness of tripolite 7sp4 gr41 #a-a4e7 is due to the facts that opal is a light mineral 7sp4 gr41 #a4i-b4b71 and that many of the shells are hollow4 ,tripolite is a good example of a soft rock composed of a hard mineral2 and it owes its value as a polishing material to the fact that it consists of a hard mineral in an exceedingly fine state of division4 ,tripolite1 when pure1 is snow-white2 but it is rarely pure1 being commonly either argillaceous or calcareous4 ,this rock is now forming in thousands of places1 in both fresh water and the ocean4 ,flint and ,chert4--,during the course of geological time1 beds of tripolite are gradually consolidated1 chiefly by percolating waters1 which are constantly dissolving and re-depositing the silica2 and1 finally1 in the place of a soft1 earthy rock1 we get a hard1 flinty one1 which we call flint if it occurs in the newer1 or chert if it occurs in the older1 geological formations4 ,besides forming beds of nearly pure silica1 which we call tripolite1 the microscopic siliceous organisms are diffused more or less abundantly through other rocks1 especially chalk and limestone4 ,in such cases the consolidation of the silica implies its segregation also2 i4e41 the silica dissolved by percolating water is deposited only about certain points in the rock1 building up rounded concretions or nodules4 ,thus1 a siliceous limestone becomes1 by the segregation of the silica1 a pure limestone containing nodules of chert1 which are usually arranged in lines parallel with the stratification4 ,specimen #af is a fragment of a flint-nodule from the chalk-formation of ,england4 ,siliceous ,tufa4--,hot water1 and especially hot alkaline water1 circulating through the earth's crust1 is always charged with silica dissolved out of the rocks2 and when such water issues on the surface in a hot spring or geyser1 it is cooled by contact with the air1 its solvent power is diminished thereby1 and a large part of the silica is deposited around the outlet as a snowy-white porous material called siliceous tufa4 ,silica deposited in this way is1 like organic silica1 always in the form of opal4 ,siliceous tufa is distinguished from clay1 slate1 chalk1 and limestone by the same tests as tripolite1 and from tripolite itself by the absence of microscopic organisms4 ,novaculite4--,through the action of percolating water and pressure1 siliceous tufa1 like tripolite1 becomes harder and denser and is thus changed to novaculite1 which holds the same relation to chemically deposited silica that chert and flint do to organically deposited silica4 ,the white novaculite obtained at the ,hot ,springs of ,arkansas1 and commonly known as ,arkansas stone1 is a typical example of this rock4 ,the rock which1 on account of the use to which it is put1 is known as buhr-stone1 is also an excellent example of chemically deposited silica4 ,it is usually somewhat porous and fossiliferous4 7#d7 ,calcareous ,group4--,these are the lime-rocks1 including the carbonate of lime1 in limestone and dolomite1 the sulphate of lime1 in gypsum1 and the phosphate of lime1 in phosphate rock4 ,these rocks are even more closely connected in origin than in composition2 and it is for this reason that rock-salt1 which of course contains no lime1 is also included in this group4 ,limestones are formed abundantly in the open sea1 through the accumulation of shells and corals2 but when portions of the sea become detached from the main body and gradually dry up1 like the ,dead ,sea and ,great ,salt ,lake1 dolomite1 gypsum1 and rock-salt are deposited in succession as chemical precipitates4 ,phosphate rock may be regarded as a variety of limestone1 resulting from the accumulation of the skeletons and excrement of the higher animals4 ,limestone4--,this is the lithologic or rock form of carbonate of lime or calcite1 and one of the most important1 interesting1 and useful of all rocks4 ,although so simple in composition1--calcite being the only essential constituent1--limestone embraces many distinct varieties1 and is really equivalent to a whole family of rocks4 ,a highly fossiliferous limestone1 such as specimen #ch1 is1 perhaps1 the best variety with which to begin the study of the species4 ,the softness of the fossil shells of which the rock is so largely composed1 and the fact that they effervesce readily with dilute acid1 proves that they are still carbonate of lime2 and by applying the acid more carefully1 it can be seen that the compact matrix of the rock also effervesces1 consisting of shells more finely broken or comminuted and mixed with more or less clay and other impurity1 almost the entire rock being of organic origin4 ,on the coast of ,florida1 and in many other places1 we find beautiful examples of shell-limestone now in process of formation4 ,these are at first very open and porous1 because the interstices between the nearly entire shells are not yet filled up with smaller fragments and sand4 ,but when that is done1 we shall have a rock similar to the old fossiliferous limestone4 ,specimen #cg4 ,the shells and fragments1 and the grains of calcareous sand1 are1 as a rule1 quickly cemented together by the deposition of carbonate of lime between them2 so that limestone is nowhere observed occurring abundantly in an unconsolidated form4 ,limestone1 as a rule1 is not distinctly stratified in hand-specimens1 but of course that it is a true sedimentary rock is abundantly proved by the fossils2 and it goes almost without saying that limestone1 being necessarily mainly composed of organic remains1 must be to a greater extent than any other rock the great store-house of fossils2 and in no other rock are the fossils so well preserved and perfect as in limestone4 ,nevertheless1 there are extensive formations of limestone containing no discernible traces of fossils4 ,and some of these non-fossiliferous limestones1 too1 are of very recent formation4 ,some of the modern coral-reefs1 for example1 are composed of limestone which was formed only yesterday1 as it were1 and which1 from its mode of formation1 must consist entirely of corals2 and yet it shows no trace of its organic origin1 but is perfectly compact1 or1 possibly1 crystalline4 ,this frequent obliteration of the organic remains1 as well as the perfect consolidation of the rock1 is attributed to its solubility4 ,the calcium carbonate is gradually dissolved by the water1 and then deposited in the interstices in other parts of the rock4 ,specimen #ci is that variety of limestone known as chalk4 ,it is soft and earthy1 resembling both clay and tripolite1 but differing from the former in lacking the distinct argillaceous odor1 and from both by its lively effervescence with acids4 ,it appears to be entirely destitute of organic remains1 but this is a defect of our vision and not of the rock2 for1 like the tripolite1 it often appears under the microscope to be little else than a mass of shells4 ,tripolite is a deposit built up of the siliceous shells of ,diatoms and ,radiolaria1 while chalk is chiefly composed of the similar but calcareous shells of ,foraminifera4 ,our specimen is from the ,cretaceous formation of ,england2 but we have good reason to believe that chalk is now forming on a very extensive scale4 ,there are millions of square miles in the deeper parts of the ocean where the dredge brings up little else but a perfectly impalpable1 gray1 calcareous slime or ooze4 ,when examined microscopically1 this is seen to be composed chiefly of ,foraminifera shells1 and among these the genus ,globigerina predominates2 so that the deposit is frequently called ,globigerina ooze4 ,now this gray1 calcareous ooze1 when dried and compacted by pressure1 makes a soft1 white rock which can scarcely be distinguished from chalk2 in fact1 it is a modern chalk4 ,and there seems no good reason to doubt that the deposition of chalk has gone on continuously since ,cretaceous time--for several millions of years at least4 ,specimen #dj is also a white rock1 easily scratched with the knife1 and effervescing freely with acid1 and therefore a variety of limestone4 ,but its texture is very different from the other varieties we have studied4 ,it has a sparkling surface1 which we explain by saying that the rock is crystalline4 ,it is1 in fact1 a mass of minute crystals of calcite4 ,the crystalline limestones have not always been crystalline1 but it is safe to assume that they were originally entirely uncrystalline1 and in many cases rich in fossils2 but the fossils have been mainly obliterated by the crystallization4 ,crystallization generally in rocks is an indication of great age1 so that we usually say crystalline rocks must be older than uncrystalline rocks of the same composition2 and this is mainly true with the limestones4 ,when the crystallization is rather fine1 as in our specimen1 resembling granulated sugar1 we have what is commonly called saccharoidal limestone4 ,this is the typical marble4 ,marble is not a scientific name1 and the term is usually applied to any calcareous rock which will take a polish1 and sometimes even to rocks which are not calcareous at all4 ,in the section on dynamical geology1 we learned that the carbonate of calcium or calcite is deposited from the sea-water1 and limestones formed1 in two ways3 first1 in a purely chemical way1 where the water becomes saturated with calcite2 and1 second1 organically1 where the calcium carbonate is taken from the water by marine organisms to form their shells and skeletons1 and the gradual accumulation of these on the ocean-floor builds up a limestone4 ,as before stated1 the difference between these two methods of deposition is not so great as it often seems1 because we know that the animals never make the carbonate of calcium which they secrete1 but it comes into the sea ready made with the drainage from the land4 ,the limestones forming at the present time are almost wholly organic2 but the rock known as calcareous tufa is an exception4 ,this is formed under the same general conditions as siliceous tufa1 but much more abundantly1 and in cold water as well as warm2 because calcite is far more soluble 7especially in water containing carbon dioxide7 than opal or quartz4 ,it is deposited1 not only around the mouths of springs1 but also along the beds of the streams which they form1 enveloping stones1 roots1 grasses1 etc41 and building up usually a loose1 spongy mass having a very characteristic turfaceous texture4 ,the principal accessory minerals occurring in limestone are3 7#a7 kaolin1 forming argillaceous or slaty limestone1 which may be recognized by the argillaceous odor and dark color2 7#b7 quartz1 forming siliceous or cherty limestone1 known by its hardness or by the nodules of flint or chert2 7#c7 dolomite1 forming dolomitic or magnesian limestone1 which effervesces less freely with acid2 and 7#d7 serpentine1 forming serpentinic limestone1 which is sharply distinguished by the green grains of serpentine mingled with the white calcite4 ,a concretionary texture is common with limestone4 ,if the concretions are small1 like mustard-seed1 we call the rock olite2 if larger1 like peas1 pisolite4 ,dolomite4--,if for calcite1 which is the sole essential constituent of all limestone1 we substitute the allied mineral dolomite1 we have the rock dolomite4 ,as might be inferred from its composition1 dolomite is very closely related to limestone1 although there are some important differences4 ,physically1 the two rocks differ about as the two minerals do4 ,dolomite is harder than limestone1 and being also less soluble1 it resists the action of the weather more4 ,dolomite1 if pure1 effervesces feebly1 or not at all1 with cold dilute acid4 ,here1 however1 we have to recognize the fact that dolomite is rarely pure2 but there exists1 in consequence of the admixture of calcite1 a perfectly gradual passage from pure dolomite to pure limestone1 and parallel with this every degree of vigor in the reaction with acid4 ,hence1 it is entirely an arbitrary matter as to where we shall draw the line between dolomitic limestone and calcareous dolomite4 ,dolomite is a very much less abundant rock than limestone1 and1 unlike limestone1 it rarely contains many fossils1 and is never of organic origin2 i4e41 there are no organisms which secrete the mineral dolomite to form their hard parts or skeletons4 ,like gypsum and rock-salt1 dolomite is probably never deposited in the open ocean1 but only in closed basins4 ,like limestone1 it occurs with both the compact and the crystalline textures4 ,gypsum4--,when pure1 this rock 7specimen #cf7 is identical with the mineral gypsum 7specimen #ag71 except that it is rarely crystalline4 ,it is usually1 however1 not only perfectly compact1 but more or less dark-colored from the admixture of clay and other impurities4 ,its most notable characteristics are its softness1 the absence of the argillaceous odor1 except where it contains much clayey impurity1 and its non-effervescence with acids4 ,the first two usually serve to distinguish it from slate1 while the acid test separates it readily from limestone and all other carbonate rocks4 ,the deposition of gypsum is purely chemical1 and it occurs under about the same physical conditions as the deposition of salt2 i4e41 in drying-up portions of the sea4 ,hence we usually find gypsum associated with beds of rock-salt2 and1 since drying-up seas are few in number1 and small compared with the whole extent of the ocean1 we can easily understand why neither rock-salt nor gypsum are abundant rocks1 except in a few localities4 ,rock-,salt4--,this interesting and useful rock1 as we have already learned1 is deposited in a purely chemical way1 and only in drying-up portions of the sea1 like the ,dead ,sea1 ,great ,salt ,lake1 etc4 ,in some parts of ,europe there are beds of solid rock-salt over a hundred feet thick4 ,phosphate ,rock4--,although not specially abundant or attractive1 this rock is of great economic interest and importance on account of its extensive use as a fertilizer4 ,under the general head of phosphate rock are included3 7#a7 the typical guano1 which is the consolidated excrement of certain marine birds inhabiting in great numbers small coral islands in the dry or rainless regions of the tropics2 7#b7 the underlying coral rock1 which is often changed to phosphate rock through the percolation of the rain-water falling on the guano2 7#c7 accumulations of the bones and coprolites of the higher animals2 7#d7 phosphatic limestones from which the carbonate of lime has been largely dissolved away1 leaving the more insoluble phosphate of lime4 7#e7 ,metamorphic ,group 7stratified silicates74--,all the chemically and organically formed rocks which we have studied up to this point are simple1 i4e41 they consist each of only one essential mineral2 but most of the rocks in this great group of silicates are mixed1 or consist each of several essential minerals4 ,quartz is the only important constituent of these rocks which is not1 strictly speaking1 a silicate1 but in a certain sense it is also not an exception1 since it may always be regarded as an excess of acid in the rock4 ,this group of stratified rocks composed of silicate minerals is of exceptional importance1 first1 on account of the large number of species which it includes1 and1 second1 on account of the vast abundance of some of the species4 ,these are1 above all others1 the rocks of which the earth's crust is composed4 ,with unimportant exceptions1 all the rocks of this group are crystalline2 and they constitute the principal part of what is generally included under the term metamorphic rocks--a general name for all stratified rocks which have been so acted upon by heat1 pressure1 or chemical forces as to make them crystalline4 ,although the crystalline limestone1 dolomite1 iron-ores1 etc41 show us that metamorphic rocks are not wanting in the other groups4 ,as already explained1 the metamorphic or crystalline stratified rocks are usually older than the corresponding uncrystalline rocks2 but a point of greater importance here is this3 the development in the silicate rocks of crystalline characters has usually made it impossible to determine the method of their deposition1 whether mechanical or chemical4 ,in a few cases1 as with the rock greensand1 we know that the deposition is chemical2 while it is equally certain that such common silicate rocks as gneiss1 mica schist1 and many others1 often result from the crystallization of ordinary mechanical sediments1 like sandstone and conglomerate4 ,we classify all these rocks as of chemical origin1 however1 without considering the mode of their deposition1 because the subsequent crystallization is itself essentially a chemical process2 and that justifies us in saying that these rocks are made what they now are chiefly by the action of chemical forces4 ,whatever they were originally1 they have become1 through their crystallization1 rocks having a definite mineral composition which can be classified chemically4 ,some of the details of the classification of this group1 as shown in the table1 require explanation4 ,in studying the silicate minerals it was stated to be important to recognize two classes--the acidic and the basic--the dividing line falling in the neighborhood of #fj per cent4 of silica4 ,this division is important simply because ,nature has in a great degree kept the acidic and basic minerals separate in the rocks2 and few things in lithology are more important than the distinction of the silicate rocks in which acidic minerals predominate from those in which basic minerals predominate4 ,the amount of silica which any rock of this group contains is shown at a glance by the chart4 ,the vertical broken lines1 with the figures at the top1 indicate the proportion of silica1 which increases from #cj per cent4 on the right to #he per cent4 on the left2 so that the percentage of silica which a rock contains determines its position1 the acidic species being on the left1 and the basic on the right4 ,as most of these rocks are composed of two or more minerals mixed in very various proportions1 there is usually a wide range in the percentage of silica which the same species may contain2 and this is expressed in each case by the length of the dotted line under the name of the rock4 ,thus1 in syenite1 the silica ranges from #ee per cent4 to #fe per cent4 ,the horizontal line in the chart separates the gneisses1 containing feldspar as an essential constituent1 from the schists1 in which feldspar is wanting1 except as an accessory constituent4 ,we will take up the gneisses first4 ,gneiss4--,this is the most important of all rocks4 ,it forms not far from one-half of ,new ,england1 and a very large proportion of the earth's crust4 ,the name 7pronounced same as nice7 is known to have originated among the ,saxon miners1 but its precise derivation is lost in obscurity4 ,to find out what this very important rock is1 we will consult specimen #da4 ,the first glance shows us that it is not1 like the rocks we have just been studying1 composed of a single mineral1 but of several minerals1 the most conspicuous of which is the pink feldspar--orthoclase4 ,this we recognize as a feldspar3 7#a7 by its hardness1 which is a little less than that of quartz1 and distinguishes it from calcite1 a mineral having the general appearance of feldspar2 7#b7 by its color1 which separates it from hornblende and augite2 and 7#c7 by its cleavage1 which distinguishes it easily from quartz4 ,finally1 we know it is orthoclase1 and not plagioclase1 by its general aspect1 and by its association with an abundance of quartz1 which is the next most important constituent of the rock4 ,the quartz is less abundant than the orthoclase1 and more easily overlooked1 yet anyone familiar with the mineral will not fail to recognize it4 ,it forms small1 irregular1 glassy grains1 entirely devoid of cleavage1 and scratching glass easily4 ,on weathered surfaces of the rock the orthoclase becomes soft and chalky1 while the quartz remains clear and hard1 and then the two minerals are very easily distinguished4 ,besides these1 there are numerous black1 thin1 glistening scales1 which we can easily prove to be elastic1 and recognize as mica4 ,in most books on the subject1 these three minerals--orthoclase1 quartz1 and mica--are set down as the normal or essential constituents of gneiss4 ,but it is now recognized by the best lithologists that we may have true gneiss without any mica2 or we may have hornblende in the place of mica4 ,quartz and orthoclase are the only essential constituents of gneiss2 and when these alone are present1 we have the variety known as binary gneiss4 ,the addition to these essential constituents of mica1 gives micaceous gneiss2 and of hornblende1 hornblendic gneiss4 ,of these three principal varieties1 the micaceous gneiss is by far the most common and important4 ,the mica may be either the white species1 muscovite1 or the black species1 biotite2 but it is usually the former4 ,orthoclase is the predominant constituent in all typical gneiss1 usually forming at least one-half of the rock4 ,the orthoclase may1 however1 be replaced to a greater or less extent by albite1 or even by oligoclase4 ,but we frequently see the term gneiss carelessly1 or ignorantly1 applied to rocks which are destitute of feldspar1 though having the general aspect of gneiss4 ,augite rarely occurs in gneiss2 and hence1 when we observe a gneiss containing a black mineral which we know is either augite or hornblende1 it is pretty safe to call it the latter4 ,mica and hornblende1 although the principal1 are not the only1 accessory minerals in gneiss2 but the following species are also of common occurrence3 garnet1 cyanite1 tourmaline1 fibrolite1 epidote1 and chlorite4 ,gneisses1 as the table indicates1 exhibit a wide range in the proportion of silica which they contain1 varying from #fj to #he per cent42 and there is a concomitant variation in specific gravity1 from about #b4e in the most acidic to #b4h in the most basic varieties4 ,that gneiss is a true1 stratified rock is very clearly shown in specimen #da2 but1 unfortunately1 the stratification is not always so evident as in this case4 ,the mica-scales1 it will be observed1 lie parallel with the stratification1 and assist very materially to make it visible2 and gneisses containing little or no mica1 as well as some that are rich in mica1 frequently appear almost or quite unstratified4 ,these obscurely stratified varieties are commonly known as granitoid gneiss1 having the texture and general aspect of granite4 ,the sedimentary origin of gneiss is also clearly proved by its interstratification with undoubted sedimentary rocks1 such as limestone1 iron-ores1 graphite1 quartzite1 etc4 ,syenite4--,this is a much abused term1 but1 as now employed by the best lithologists1 it is the name of a rock having a single essential constituent1 viz41 orthoclase4 ,syenite in its simplest variety contains nothing but orthoclase2 but in addition we usually have either hornblende1 forming hornblendic syenite1 or mica1 forming micaceous syenite4 ,syenite1 it will be observed1 is equivalent to gneiss with the quartz removed2 but1 while gneiss is the most abundant of all rocks1 syenite is a comparatively rare rock2 and this is simply another way of saying that nearly all orthoclase is associated with quartz4 ,by admixture of quartz we get a perfectly gradual passage from syenite to gneiss4 ,the orthoclase in syenite is more frequently replaced by plagioclase than it is in gneiss4 ,in syenite1 too1 hornblende is much more abundant than mica2 although just the opposite is true in gneiss4 ,and1 again1 in gneiss the mica is principally muscovite2 but in syenite it is almost exclusively biotite4 ,augite is a common accessory in the more basic syenite2 but garnet1 tourmaline1 and the other accessory minerals1 occurring so frequently in gneiss1 are almost unknown in syenite4 ,the specific gravity of syenite varies from #b4g to #b4i4 ,diorite4--,this is a more important rock than syenite2 but it is of analogous1 though more basic1 composition1 containing a single essential constituent1 viz41 plagioclase4 ,any of the triclinic feldspars may occur in this rock1 but oligoclase is most common4 ,like syenite1 diorite usually contains hornblende1 often in large proportion1 forming hornblendic diorite1 which sometimes passes into rocks composed entirely of hornblende4 ,it also1 but less frequently1 contains mica1 forming micaceous diorite4 ,the mica is usually biotite1 rarely muscovite4 ,mica and hornblende also often occur together in diorite1 and the same is true of syenite and gneiss4 ,quartz is of common occurrence in the more acidic varieties of diorite1 and augite in the more basic4 ,this is a good example of a basic rock1 for all its normal constituents are basic2 but the percentage of silica varies from #de in those varieties richest in labradorite and augite to #fj or more in those containing more or less quartz and orthoclase4 ,there is a corresponding change of color from dark to light1 and of specific gravity from #b4g to #c4a4 ,diorite is not rich in accessory minerals2 besides those already mentioned1 the most important are chlorite1 epidote1 pyrite1 and magnetite4 ,few rocks are more clearly stratified than diorite1 whether we consider the hand-specimen1 or its relations to other formations4 ,it is an abundant rock in ,new ,england4 ,norite4--,like diorite1 this is essentially a plagioclase rock2 but there are1 nevertheless1 important differences4 ,the plagioclase in diorite is mainly the more acidic species1 like oligoclase2 while in norite the more basic species1 such as labradorite and anorthite1 predominate4 ,hornblende1 which we have observed to be an important and rather constant constituent of diorite and syenite1 is much less abundant in norite2 but its place is taken by augite and the allied minerals1 hypersthene and enstatite4 ,black mica is common in norite2 but white mica1 orthoclase1 and quartz rarely occur4 ,norite is the most basic of all the feldspathic rocks1 as gneiss is the most acidic2 while syenite and diorite stand as connecting links1 forming a gradual passage between the two extremes4 ,thus1 in passing from gneiss to norite1 we have observed a gradual diminution of the quartz1 a gradual change in feldspar from orthoclase to the most basic plagioclase2 at first a gradual increase in hornblende1 and then a gradual change from hornblende to augite2 and1 finally1 a gradual substitution of black mica for white4 ,the amount of silica has decreased over #dj per cent42 and the specific gravity has increased from #b4e in the lightest gneiss to at least #c4b in the heaviest norite4 ,we have also passed from light colored rocks to dark2 and from those resisting atmospheric action to those easily decomposed4 ,the most characteristic accessory constituents of norite1 besides those already mentioned1 are magnetite and chrysolite2 though garnet1 serpentine1 and pyrite often occur4 ,in texture1 this rock varies from compact to very coarsely crystalline4 ,the specimen of labradorite 7,no4 #bc71 from the norite of ,labrador1 affords some idea of the coarseness of the crystallization in much of this rock4 ,it is not a common rock1 except in certain regions1 the best known of which in eastern ,north ,america are the coast of ,labrador1 various points in ,canada north of the ,st4 ,lawrence1 and the eastern border of the ,adirondack ,mountains4 ,in hand-specimens1 norite rarely appears stratified2 but in the solid ledges the stratification is often as distinct as could be desired4 ,many lithologists call the rocks here designated norite gabbro1 and class them all in the eruptive division as essentially a coarse variety of diabase4 ,in a similar manner1 diorite and syenite are denied a place in the sedimentary series4 ,but the stratified plagioclase rocks seem to have as strong a claim to recognition as gneiss4 ,we turn now to the important and interesting division of the non-feldspathic rocks or schists4 ,mica ,schist4--,this is1 next to gneiss1 the most abundant rock in ,new ,england4 ,specimen #dc is a typical example1 and from it we can readily learn what mica schist is4 ,a glance suffices to show that it is chiefly composed of mica1 but not entirely2 for1 on carefully examining the edges of the specimen1 we cannot fail to see thin layers of hard1 glassy quartz interwoven with the mica4 ,the quartz layers are short and overlapping1 and we have here a good illustration of the schistose texture2 this is1 in fact1 a typical schist4 ,mica schist usually consists1 as in this instance1 of mica and quartz2 but it may be composed of mica alone2 and sometimes kaolin or clay takes the place of the quartz1 forming argillaceous mica schist4 ,the mica in the latter is usually in very fine scales and rather inconspicuous1 and the rock often passes into ordinary clay slate4 ,similarly1 when the mica becomes deficient in the quartzose mica schist1 a passage into ordinary quartzite is the result4 ,a little feldspar is sometimes present in the rock1 which thus passes into micaceous gneiss4 ,specimen #dc contains several crystals of red garnet1 giving the variety garnetiferous mica schist4 ,there is no other rock that contains such a large variety of beautiful accessory minerals as mica schist2 and for the mineralogist it is one of the most attractive rocks4 ,few rocks are more distinctly stratified2 and the stratification can usually be observed in hand-specimens4 ,the mica in these rocks may be either muscovite or biotite1 or both2 but the former is most common4 ,no rock shows a greater variation in the percentage of silica which it contains than mica schist1 as we pass from varieties which are nearly all quartz to those which are nearly all mica4 ,closely related to mica schist is the rock now known as hydromica schist1 in which the ordinary anhydrous micas are replaced by hydromica4 ,it is distinguished from mica schist by being somewhat softer1 less harsh to the touch1 and less lustrous4 ,it is to be regarded usually as an incipient mica schist1 which has not yet become anhydrous2 though it may sometimes be just the reverse2 viz43 an old mica schist which has become hydrated through the action of meteoric waters4 ,it contains fewer accessory minerals than mica schist4 ,hornblende ,schist4--,this is a stratified aggregate of hornblende and quartz4 ,the quartz is granular and in thin layers1 as in mica schist2 but the micaceous structure is wanting1 and consequently the rock does not cleave readily in the direction of the bedding4 ,the hornblende is mostly finely crystalline1 but sometimes occurs in large1 bladed crystals4 ,garnet and some other minerals are of common occurrence in the rock2 but it is not rich in accessories like mica schist4 ,the chief difficulty in recognizing this rock consists in determining whether the white mineral is all quartz or partly feldspar4 ,in the latter case1 of course1 it becomes a hornblendic gneiss4 ,amphibolite 7,hornblende ,rock74--,this is the name applied to a rock having hornblende as its sole essential constituent4 ,hornblende schist sometimes passes into amphibolite1 through the absence of quartz2 and so does diorite1 when the feldspar is deficient or wanting4 ,specimen #bj1 though small1 is a typical example of this rock4 ,the physical and chemical characteristics are essentially the same as for the mineral hornblende4 ,the texture varies from coarsely to finely crystalline4 ,the crystals are usually short and thick1 and lie in all directions in the rock1 which is thus very massive1 the schistose texture being entirely wanting1 and the stratification rarely showing in small masses4 ,biotite is a common accessory in amphibolite1 and garnet and magnetite frequently occur4 ,by the substitution of augite for hornblende1 in the description of amphibolite1 we get the much rarer1 but otherwise very similar1 rock1 pyroxenite4 ,talc ,schist 7,steatite or ,soapstone74--,although not abundant1 this is a useful and familiar rock4 ,the composition is implied in the name2 and by comparing it with the specimen of talc 7,no4 #eh7 we can readily see that they are essentially identical4 ,typical talc schist is pure talc2 but the talc is often mixed with more or less quartz or feldspar2 and mica1 chlorite1 hornblende1 garnet1 and other minerals are of common occurrence4 ,this rock embraces two distinct varieties1 the massive and the schistose1 or foliated4 ,the former is the common soapstone 7specimen #ga71 which is a confused mass of crystals lying in all directions1 and with no visible stratification in the small mass4 ,in the latter1 as in specimen 1 the talc scales lie in parallel planes1 giving the rock a micaceous structure1 and causing it to split easily in the direction of the stratification4 ,the cleavage surfaces are often wavy or corrugated2 and the same is true of all schistose rocks4 ,talc schist is easily distinguished from all other rocks by its light-grayish or greenish color1 combined with its extreme softness1 and its smooth1 slippery feel4 ,chlorite ,schist4--,the one essential constituent of this rock is chlorite1 and the mineral specimen 7,no4 #bf7 answers equally well as an example of the rock4 ,as with talc schist1 quartz1 feldspar1 and hydromica are rarely entirely absent4 ,besides these1 the principal accessories are hornblende1 magnetite1 garnet1 and epidote4 ,this rock also agrees with talc schist in presenting two principal varieties1 the massive and the schistose4 ,it is easily distinguished from talc schist by its darker color and streak1 which are very characteristic2 while its green color1 softness1 and unctuous feel separate it from all other rocks4 ,this is the most basic of all the silicate rocks2 but1 in consequence of containing a large proportion of water1 it is not the heaviest4 ,it is1 in fact1 interesting and important to observe that all these hydrous silicate rocks--talc schist1 chlorite schist1 greensand1 and serpentine--are distinctly lighter in each case than anhydrous rocks containing the same proportion of silica4 ,they are also notable1 as a class1 for their softness1 smooth feel1 and green color4 ,serpentine4--,as the name implies1 this rock is simply the mineral serpentine occurring in large masses1 and its characteristics are precisely the same4 ,it is fine-grained1 massive1 compact1 rather soft1 but very tough1 and varies in color from very dark green to light greenish-yellow4 ,the dark colors predominate1 and specimen #be is a typical example4 ,serpentine is often intimately associated with limestone and dolomite4 ,the white veins running irregularly through the variety known as ,verd ,antique ,marble1 however1 are not calcite1 as commonly supposed1 but magnesite4 ,they do not effervesce freely with cold1 dilute acid1 for the entire rock is magnesian1 and it is probable have been at one time simply cracks along which water holding carbon dioxide has penetrated1 changing the magnesia from a silicate to a carbonate4 ,geologists were1 at one time1 almost unanimous in the opinion that all serpentine is of eruptive origin2 but now it is conceded by the great majority to be in some cases a sedimentary rock4 ,it is found interstratified with gneiss1 limestone1 all the schists1 and many other stratified rocks4 ,when occupying the position of an eruptive it is never an original rock2 but has been formed by the alteration1 in situ1 of some basic anhydrous rock1 most commonly olivine basalt4 ,greensand4--,this rock 7specimen #bg7 consists chiefly of the mineral glauconite1 mingled usually with more or less sand1 clay1 or calcareous matter4 ,it is usually very friable1 or in an entirely unconsolidated state4 ,it is most abundant in the newer geological formations1 especially the ,cretaceous and ,tertiary2 and is1 perhaps1 the only one of the stratified silicate rocks now forming on an extensive scale in the ocean4 ,its value as a fertilizer1 for which purpose it is extensively employed1 is due to the potash that it contains4 ,following is a systematic summary of the mineralogical composition of the rocks of this great division of silicates2 and this1 combined with the classification on page #fi1 presents in a condensed form all the more important facts contained in the preceding descriptions4 ,only the more constant and normal constituents of the species are enumerated in each case3-- ,names of ,species4 ,constituent ,minerals4 ---------------------------------------- ---------------- ,orthoclase and ,quartz4 ,gneiss ,orthoclase1 ,quartz1 and ,mica4 ,orthoclase1 ,quartz1 and ,hornblende4 ---------------------------------------- ---------------- ,orthoclase4 ,syenite ,orthoclase and ,hornblende4 ,orthoclase and ,mica4 ---------------------------------------- ---------------- ,plagioclase 7chiefly ,oligoclase74 ,diorite ,plagioclase and ,hornblende4 ,plagioclase and ,mica4 ---------------------------------------- ---------------- ,plagioclase 7chiefly ,labradorite74 ,norite ,plagioclase and ,augite 7,diallage74 ,plagioclase and ,mica4 ---------------------------------------- ---------------- ,mica4 ,mica ,schist ,mica and ,quartz4 ,mica and ,kaolin4 ---------------------------------------- ---------------- ,hornblende ,schist ,hornblende and ,quartz4 ---------------------------------------- ---------------- ,amphibolite ,hornblende4 ,pyroxenite ,pyroxene4 ---------------------------------------- ---------------- ,talc ,schist ,talc4 ---------------------------------------- ---------------- ,chlorite ,schist ,chlorite4 ---------------------------------------- ---------------- ,serpentine ,serpentine4 ---------------------------------------- ---------------- ,greensand ,glauconite4 #b4 ,eruptive or ,unstratified ,rocks4 ,the rocks of this great class are formed by the cooling and solidification of materials that have come up from a great depth in the earth's crust in a melted and highly heated condition4 ,when the fissures in the earth's crust reach down to the great reservoirs of liquid rock1 and the latter wells up and overflows on the surface1 forming a volcano1 then we may1 as was pointed out on page #cc1 divide the eruptive mass into two parts3 first1 that which has actually flowed out on the surface1 and cooled and solidified in contact with the air1 forming a lava flow2 second1 that which has failed to reach the surface1 but cooled and solidified in the fissure1 forming a dike4 ,lava flows or volcanic rocks and dikes or plutonic rocks are identical in composition2 but there is a vast difference in texture1 due to the widely different conditions under which the rocks have solidified4 ,the dike or fissure rocks solidify under enormous pressure1 and this makes them heavy and solid--free from pores4 ,they are surrounded on all sides by warm rocks3 this causes them to cool very slowly1 and allows the various minerals time to crystallize4 ,other things being equal1 the slower the cooling the coarser the crystallization2 and hence1 the greater the depth below the surface at which the cooling takes place1 the coarser the crystallization4 ,the volcanic rock1 on the other hand1 cools under very slight pressure2 and the steam1 which exists abundantly in nearly all igneous rocks at the time of their eruption1 is able to expand1 forming innumerable small vesicles or bubbles in the liquid lava2 and these remain when it has become solid4 ,cooling in contact with the air1 the lava cools quickly1 and has but little chance for crystallization4 ,hence1 to sum up the matter1 we say3 plutonic rocks are solid and crystalline2 and volcanic rocks are usually porous or vesicular1 and uncrystalline4 ,as we descend into the earth's crust1 it is perfectly manifest that the volcanic must shade off insensibly into the dike rocks1 and we find it impossible to draw any but an arbitrary plane of division between them2 but this is no argument against this classification1 for1 as already stated1 all is gradation in geology1 and we experience just the same difficulty in drawing a line between conglomerate and sandstone1 or between gneiss and mica schist1 as between the dike rocks and volcanic rocks4 ,we will now observe to what extent the distinctions between these two great classes of eruptives can be traced in the rocks themselves1 beginning with the dike rocks4 ,but first it is important to notice the general fact1 clearly expressed in the classification1 that1 with perhaps some trifling exceptions which need not be mentioned here1 all eruptive rocks are silicates1 and nearly all are feldspathic silicates4 ,they are of definite mineralogical composition1 and1 like the chemically and organically formed stratified rocks1 can be classified chemically4 ,but1 although there are eruptives corresponding closely in composition to the feldspathic silicates1 which we have just studied1 we find among them little to represent the non-feldspathic silicates1 and nothing corresponding in composition to the limestones1 dolomites1 gypsum1 flint1 tripolite1 siliceous tufa1 iron-ores1 bitumens or coals4 #a4 ,,plutonic 7,,dike7 ,,rocks4--,these are also known as the ancient eruptive rocks1 and for this reason3 ,it is impossible1 of course1 for us to observe them except where they occur on or near the earth's surface4 ,but1 since they are formed wholly below the surface1 and usually at great depths in the earth1 it is evident that they can appear on the surface only as the result of enormous erosion2 and erosion is a slow process1 demanding1 in these cases1 many thousands or millions of years4 ,therefore1 the more ancient dike rocks alone are within our reach2 those of recent formation being still deeply buried in the earth's crust4 ,it follows1 as a corollary to this explanation1 that the coarseness of the crystallization of any dike rock must be a rough measure of its age and of the amount of erosion which the region has suffered since its eruption4 ,as regards composition1 the dike rocks present1 as already stated1 essentially the same combinations of minerals as the feldspathic silicates of the stratified series1 but occurring under different physical conditions and having a widely different origin4 ,the only important difference in texture between the two classes of rocks is that the sedimentary rocks are stratified and the dike rocks are not2 and when we consider that the dike rocks sometimes present a laminated structure that resembles stratification1 while the sedimentary rocks frequently appear unstratified1 it is easy to understand why1 in the absence of any marked difference in composition1 geologists have often found it difficult to distinguish the two classes of rocks4 ,we also find here the explanation and the justification of the fact that the names of the dike rocks are in most cases the same as those of the sedimentary rocks of similar composition4 ,granite4--,granite 7from the ,latin granum1 a grain7 is a crystalline-granular rock1 agreeing in composition with gneiss4 ,the essential constituents are quartz and orthoclase2 and when they alone are present we have the variety binary granite4 ,mica1 however 7commonly muscovite1 sometimes biotite1 and frequently both7 is usually added to these1 forming micaceous granite 7specimen #dd72 and often hornblende1 forming hornblendic granite 7specimen #de74 ,the orthoclase is sometimes replaced in part by triclinic species1 especially albite and oligoclase4 ,accessory minerals are not so abundant in granite as in gneiss2 but1 besides those named1 garnet1 tourmaline1 pyrite1 apatite1 and chlorite are most common4 ,orthoclase is always the predominant ingredient2 and1 except when there is much hornblende present1 usually determines the color of the granite4 ,thus1 specimens #dd and #de are gray because they contain gray orthoclase2 while all red granites contain red or pink orthoclase4 ,the quartz has usually been the last of the constituents to crystallize or solidify2 and1 having been thus obliged to adapt itself to the contours of the orthoclase and mica1 it is rarely observed in distinct crystals4 ,in texture1 the granites vary from perfectly compact varieties1 approaching petrosilex1 to those which are so coarsely crystalline that single crystals of orthoclase measure several inches in length4 ,of course one of the most important things to be observed about granite1 especially in comparing it with gneiss1 is the complete absence of anything like stratification2 that1 as before stated1 being the only important distinction between the two rocks4 ,gneiss is the most abundant of all stratified rocks1 and granite stands in the same relation to the eruptive series4 ,syenite4--,this is an instance where stratified and eruptive rocks1 agreeing in composition1 have the same name4 ,that rocks consisting of orthoclase1 of orthoclase and hornblende1 or of orthoclase and mica1 i4e41 having the composition of syenite1 do occur in both the eruptive and stratified series there can be no doubt4 ,they should1 however1 have distinct names on account of their unlike origins2 and would have but for the practical difficulty in determining1 in many cases1 whether the rock is stratified or not4 ,the best that we can do now1 when we desire to be specific1 and have the necessary information1 is to say stratified syenite or eruptive syenite1 as the case may be4 ,diorite4--,here1 again1 we find identity of names1 as well as of composition1 between the two great series4 ,eruptive diorite is an abundant and well known rock1 and consists of the same minerals as stratified diorite combined in the same proportions4 ,diorite includes a large part of the dike rocks commonly known as 8trap0 and 8greenstone40 ,the principal accessories are chlorite1 epidote1 pyrite1 magnetite1 apatite1 and quartz4 ,the texture varies from perfectly compact or felsitic to coarsely crystalline2 averaging1 however1 less coarse than syenite and granite4 ,diabase4--,by referring to the classification it will be seen that diabase occupies the same position among the dike rocks as norite among the stratified rocks4 ,like norite it consists usually of the more basic varieties of plagioclase with or without augite1 diallage1 or hypersthene4 ,augite1 or one of its representatives1 is usually present1 and is often the principal constituent4 ,specimen #a shows a somewhat equal development of the feldspar and augite4 ,the name gabbro is sometimes applied to the coarser and more feldspathic diabases1 and especially to those containing diallage or hypersthene in the place of common augite4 ,in the opinion of some high authorities1 however1 it is unnecessary to recognize two species here2 and it makes the classification more simple and symmetrical not to do it4 ,the principal accessories in diabase are biotite1 chlorite1 magnetite1 pyrite1 calcite1 and olivine4 ,chlorite is often an important constituent1 giving the rock a greenish aspect2 but here1 as well as in diorite1 the chlorite is due chiefly or entirely to the alteration of the augite and feldspar2 and the chloritic varieties of diorite and diabase together make up the old species 8greenstone40 ,similarly1 the more compact and darker varieties of these two rocks1 forming regular1 wall-like dikes1 are known as 8trap40 ,specimen #df4 ,in consequence of their more basic composition1 diabase and diorite are usually strongly contrasted with granite and syenite in color and specific gravity1 being darker and heavier4 ,the basic rocks1 too1 decay much more readily than the acidic4 #b4 ,,volcanic ,,rocks4--,as regards composition1 we shall find nothing new in the volcanic series2 for the rocks of this group present essentially the same combination of minerals as the dike rocks4 ,in composition1 the dike and volcanic rocks are identical2 but in texture1 as already explained1 there is a vast difference4 ,the volcanic rocks differ so widely in texture from both the dike and stratified species1 that there is rarely any difficulty in distinguishing them2 and hence they have in every instance distinct names4 ,volcanic rocks are rarely found in this part of the world2 and specimens of most of them are difficult to obtain4 ,for this reason they can only be noticed briefly here1 since it is the plan of this ,guide to give especial attention only to those portions of the subject which can be illustrated by material within easy reach of teachers4 ,rhyolite4--,this rock corresponds in composition with granite and gneiss1 but is less frequently micaceous4 ,the orthoclase in rhyolite1 and generally in volcanic rocks1 is the clear1 pellucid variety--sanidine4 ,it is more difficult to separate from quartz than ordinary orthoclase1 the chief distinguishing feature being its cleavage4 ,plagioclase and hornblende are common1 but not abundant1 constituents4 ,the mica1 when present1 is usually biotite4 ,the texture of rhyolite is often more or less distinctly porphyritic1 having a finely crystalline or granular matrix1 with interspersed crystals of sanidine and quartz4 ,the rock has usually a rough1 harsh feel2 and while the coarser varieties have the aspect of granite1 the finer approach petrosilex2 but all are somewhat porous1 which is seen in the lower specific gravity of rhyolite as compared with granite and gneiss4 ,trachyte4--,in texture and general aspect rhyolite and trachyte are nearly identical4 ,trachyte1 however1 is darker1 contains little or no quartz1 and more hornblende and plagioclase4 ,in fact1 it agrees in composition with syenite4 ,this is one of the most important of the volcanic rocks4 ,obsidian4--,obsidian is sharply distinguished from all other rocks by its perfect vitreous texture2 it is a true volcanic glass4 ,its surface 7specimen #dg7 is smooth and glassy1 and its fracture eminently conchoidal4 ,to the naked eye1 and usually under the microscope1 the typical variety is perfectly homogeneous2 chemical analysis1 however1 shows that it has the composition1 commonly of rhyolite1 but sometimes of trachyte4 ,obsidian is1 in fact1 simply rhyolite or trachyte which1 cooling quickly1 has not had time to crystallize1 but has remained permanently in the amorphous or glassy state4 ,the composition is sometimes partially revealed where a portion of the sanidine comes out in distinct crystals porphyritically interspersed through the glass4 ,the homogeneity of the texture is sometimes disturbed3 by numerous minute concentric cracks1 forming what is known as perlitic structure and the variety perlite2 by numerous small spherical concretions1 forming the spherulitic structure and the variety spherulite2 and also by the banding1 which is the result of flowing while in a plastic state1 whereby portions of the glass of slightly different colors are drawn out into layers and interlaminated4 ,the bands are rarely continuous for any distance1 being usually merely elongated lenticular streaks4 ,the glassy state is generally one of inferior density1 and hence we find that obsidian is lighter than the crystalline rocks of the same composition4 ,obsidian is a good illustration of a non-essential color1 for its capacity and jet-black color are due entirely to impurities4 ,in very thin flakes it is transparent and white4 ,it also forms a white powder when crushed1 i4e41 it has a white streak4 ,obsidian is often vesicular1 from the expansion of the steam and other gases which it contained when liquid4 ,the most thoroughly vesicular varieties are known as pumice 7specimen #dh74 ,the vesicular texture1 by rendering the rock impervious to light1 conceals the impurities1 and thus we get a snow-white pumice from black obsidian4 ,the vesicles are frequently elongated1 sometimes in a definite direction1 though often forming an irregular net-work of glassy fibres4 ,pumice is often light enough to float on water1 and it is transported thousands of miles by the oceanic currents4 ,it is employed in the arts1 and good specimens can be obtained at almost any drug-store4 ,petrosilex and ,felsite4--,sharply defined groups are unknown in lithology1 but all is gradation2 and between rhyolite and trachyte1 which are always more or less distinctly crystalline1 and obsidian1 which is a true glass and perfectly amorphous1 there is no break4 ,it is impossible to draw a sharp line and say1 ,here the vitreous texture ends and the crystalline begins2 for the transition is not abrupt1 but gradual4 ,we recognize1 really1 in these feldspathic rocks1 an intermediate state1 which is neither crystalline nor colloid1 but both2 and this lithologists have designated the felsitic texture4 ,felsitic matter cannot1 even with the highest powers of the microscope1 be resolved into separate grains or particles2 and it does not exhibit1 except perhaps very indistinctly1 the phenomenon of double refraction4 ,in other words1 it is not truly crystalline or stony1 and yet it is just as clearly not amorphous or glassy4 ,feldspathic rocks exhibiting the felsitic texture in whole or in part are known as felsites4 ,many high authorities hold that true felsites are found only among the eruptive rocks2 while others claim that they are in part1 or wholly1 of sedimentary origin4 ,the writer accepts the former view4 ,the felsites are in part acid lavas which have cooled too slowly to form a true glass1 like obsidian1 and yet too quickly to become truly crystalline1 like rhyolite and trachyte4 ,but they are also in large part simply devitrified obsidian4 ,glass is an unstable form of mineral matter2 and every species of glass1 including obsidian1 tends with the lapse of time to become crystalline or stony1 the amorphous changing to the felsitic structure4 ,thus1 in many cases or usually1 what we now call felsites were originally true glassy obsidian4 ,being perfectly intimate mixtures of the component minerals1 the composition of felsites can usually be determined with certainty only by means of chemical analysis4 ,by this means chiefly1 it has been proved that there are felsites agreeing in composition with both rhyolite and trachyte4 ,there is this general difference in composition1 however1 between these crystalline rocks and the felsites2 viz43 mica1 hornblende1 and augite are generally wanting in the latter4 ,from this it follows that the felsites are1 with unimportant exceptions1 composed either of quartz and feldspar or of feldspar alone4 ,the physical differences between the felsites of unlike composition are not great2 but they are sufficient to warrant the division of the felsites into two species3 a basic species1 to which the term felsites may properly be restricted2 and an acidic species1 for which petrosilex is a very appropriate name4 ,according to this arrangement1 felsite is composed chiefly of orthoclase1 and1 as the table shows1 agrees in composition with trachyte2 while petrosilex consists mainly of orthoclase and quartz1 agreeing in composition with rhyolite4 ,we find here nothing new in composition2 but petrosilex and felsite are simply the crystalline rocks which we have already studied1 repeated under a different texture4 ,the typical felsite or petrosilex is composed entirely of felsitic matter1 and is perfectly homogeneous1 like flint or jasper1 which it closely resembles in hardness and other physical characteristics4 ,as a rule1 however1 the rock is not entirely homogeneous1 but there is a manifest tendency in the component minerals1 and especially in the feldspar1 to separate out1 usually in the form of crystals4 ,in the banded variety 7specimen #db7 the rock is built up of thin layers1 which are often alternately quartzose and feldspathic4 ,there is not a perfect separation of the minerals2 but that the quartz is chiefly in the dark layers1 and the feldspar in the light1 is shown by the way in which the layers are affected by the weather4 ,one of the most common varieties is where a portion1 frequently a large portion1 of the feldspar comes out in the form of distinct1 separate crystals1 producing a porphyritic texture4 ,specimens #e1 #f1 and #g are examples of porphyritic felsite2 and after examining these we can no longer doubt that feldspar is an important constituent of the rock4 ,petrosilex and felsite are more generally porphyritic than any other rocks2 and they are commonly called porphyry4 ,it is better1 however1 since almost any rock may be porphyritic1 and since this texture cannot be correlated with any particular composition1 not to use porphyry as a rock-name1 but simply as the name of a very important rock-texture4 ,the banded and porphyritic textures are about equally characteristic of petrosilex and felsite4 ,in petrosilex1 quartz1 as well as feldspar1 is sometimes porphyritically developed1 forming the variety known as quartz-porphyry4 ,there is no limit to the proportion of the quartz and feldspar which may crystallize out in this way1 and thus we find a perfectly gradual passage from normal petrosilex or felsite to thoroughly crystalline granite and syenite4 ,andesite4--,this rock has nearly the texture of rhyolite and trachyte1 but is darker and heavier1 and corresponds in composition to diorite1 consisting of plagioclase and hornblende1 with usually more or less sanidine1 quartz1 augite1 biotite1 and magnetite4 ,basalt4--,the rock bearing this familiar name represents diabase among the dike rocks4 ,it is the most basic of the volcanic rocks1 and consists of the more basic varieties of plagioclase1 especially labradorite1 with augite1 magnetite1 and titanic iron4 ,olivine is a very common and characteristic constituent1 and the plagioclase is often replaced in part by leucite and nephelite4 ,the basalts are usually black1 and of high specific gravity2 and vary in texture from compact to coarsely crystalline4 ,the contraction due to cooling frequently results in the development of a columnar structure of remarkable regularity1 the columns being normally hexagonal and standing perpendicularly to the cooling surfaces of the mass4 ,this structure occurs in other eruptive rocks1 but is most characteristic of basalt4 ,tachylite4--,tachylite is a highly basic volcanic glass1 standing in the same relation to basalt and andesite that obsidian does to trachyte and rhyolite4 ,it is much heavier than obsidian1 and is perfectly black and opaque1 except in the finest fibres4 ,it is a comparatively rare rock1 for the reason that basalt and andesite crystallize more readily than the acidic rocks on passing from the liquid to the solid state4 ,on the surface of the basic lava1 however1 where it is in contact with the air1 and congeals almost instantly1 a film of glass is formed2 but this may not be more than a small fraction of an inch in thickness4 ,like obsidian1 tachylite is often vesicular2 but the vesicular basic rocks1 as well as the solid1 are usually stony4 ,they occur in vast abundance in many volcanic regions1 and may be considered the typical lava 7specimen #di74 ,in the more ancient lavas1 the vesicles are frequently filled by various minerals--chlorite1 epidote1 quartz1 calcite1 etc4--deposited by infiltrating waters1 and derived in most cases from the decomposition of the original constituents of the rock4 ,thus the vesicular is changed to the amygdaloidal texture1 and the lava becomes an amygdaloid 7specimen #ej74 ,the amygdaloidal texture is common in the basic lavas and rare in pumice1 simply because the former are more readily decomposed and contain a greater variety of bases from which secondary minerals can be formed4 ,porphyrite and ,melaphyr4--,these two rocks hold essentially the same relation as regards origin and structure to the basic lavas that petrosilex and felsite do to the acidic lavas4 ,porphyrite agrees in composition with andesite1 and melaphyr with basalt4 ,they are usually dark-colored rocks having a compact or felsitic texture4 ,porphyrite is1 as the name implies1 very commonly porphyritic2 while melaphyr is often vesicular or brecciated1 exhibiting all the structural features of tachylite and basalt1 and being in its older forms very generally amygdaloidal4 ,volcanic ,tuff and ,agglomerate4--,besides the crystalline1 glassy1 and felsitic lavas1 already described1 and due chiefly to the rate of cooling of the liquid rock1 we may recognize a fourth class to include the very abundant lavas which1 during explosive eruptions1 are ejected in the solid state1 being violently blown out of the crater in the form of dust and fragments4 ,falling on the slopes of the volcano or over the surrounding country1 as in the case of the buried city of ,pompeii1 the fragmental lavas remain largely unstratified4 ,but when1 as frequently happens1 they fall into the sea1 they are assorted by the waves and currents and arranged in layers after the manner of ordinary sediments1 with which they are often more or less mixed4 ,before they become consolidated the finer fragmental lava1 of whatever composition1 is called volcanic dust1 and the coarser lapilli or volcanic sand2 while the consolidated materials are known as tuff and agglomerate respectively4 ---------------------------------------- -------------------------------- ,,supplement ,,to ,,lithology4 ,illustration ,,vein ,,rocks4 ,all rocks are not embraced in the sedimentary and eruptive divisions1 but there is a third grand division1 which1 although rarely mentioned or recognized in the more comprehensive works on geology1 it is deemed best not to leave entirely unnoticed here4 ,these are the vein rocks4 ,they present an immense number of varieties1 and yet1 taken altogether1 form but a small fraction of the earth's crust4 ,they are1 however1 the great repositories of the precious and other metals1 and hence are objects of far greater interest to the miner and practical man than the eruptive rocks1 or1 in some parts of the world1 even than the sedimentary rocks4 ,the vein rocks1 like the eruptive rocks1 occupy fissures in the earth's crust intersecting the stratified formations2 but the fissures filled with vein rocks are called veins1 and not dikes4 ,we will first notice the mode of formation of a typical vein1 and then examine its contents4 ,geologists are agreed that water penetrates to a very great depth in the earth's crust4 ,all minerals are more or less soluble in water2 and we may consider the water circulating through the rocks1 especially at considerable depths1 as1 in most cases1 a saturated solution of the various minerals of which they are composed4 ,very slight changes in the conditions will cause saturated solutions to deposit part of their mineral load4 ,the water at great depths has a high temperature1 and is subjected to an enormous pressure2 and both of these circumstances favor solution4 ,suppose1 now1 that these hot subterranean waters enter a fissure in the crust and flow upwards1 perhaps issuing on the surface as a warm mineral spring2 as they approach the surface1 the temperature and pressure1 and consequently their solvent power1 are diminished2 and a portion of the dissolved minerals must be deposited on the walls of the fissure1 which thus becomes narrower1 and in the course of time is gradually filled up4 ,the vein is then complete2 and the mineral waters are forced to seek a new outlet4 ,veins have the same general forms as dikes1 since the fissures are formed in the same way for both2 but the vein is of slow growth1 and may require ages for its completion1 while the dike is formed in an hour or a day4 ,it is now generally believed that water is an important agent in the formation of eruptive rocks2 since they all contain water at the time of their eruption2 and since it has been demonstrated that1 while ordinary rocks require a temperature of #bjjj to #cjjj for their fusion in the absence of water1 they are liquified at temperatures below #ajjj in the presence of water4 ,in other words1 common rocks are very infusible and insoluble bodies1 and heat and water acting independently have little effect upon them2 but when fire and water are combined in what is now known as aqueo-igneous fusion1 they prove very efficient agents of liquefaction4 ,if we adopt these views1 then it can be shown that1 in origin1 veins and dikes differ in degree only1 and are not fundamentally unlike2 and the formation and relations of the three great classes of rocks may be summarized as follows3-- ,the ocean and atmosphere1 operating on the earth's surface1 have worked over and stratified the crust1 until the sedimentary rocks have now an average thickness variously estimated at from ten to thirty or forty miles4 ,this entire thickness of stratified rocks1 and a considerable depth of the underlying unstratified crust1 must be saturated with water2 and all but the more superficial portions of this water-soaked crust must be very hot1 the temperature increasing steadily downwards from the surface4 ,both eruptive and vein rocks originate in this highly heated1 hydrated crust4 ,eruptive rocks are formed when the heat1 aided by more or less water1 softens the rocks1 either stratified or unstratified1 by aqueo-igneous fusion1 and the plastic materials are forced up through fissures to or toward the surface4 ,vein rocks are formed when the water1 aided by more or less heat1 dissolves the rocks1 either stratified or unstratified1 by what may be called igneo-aqueous solution1 and subsequently deposits the mineral matter in1 i4e41 on the walls of1 fissures leading up to or toward the surface4 ,in the case of the dike rocks1 heat is the chief agent1 and water merely an auxiliary2 while with the vein rocks it is just the reverse4 ,but between the two it is probably impossible to draw any sharp line4 ,the water circulating through the crust1 and saturated with its various mineral constituents1 has been called the 8juice0 of the crust2 and veins are formed by the concentration of this earth-juice in fissures4 ,one of the most important characteristics of the vein rocks1 as a class1 is the immense variety which they present2 for nearly every known mineral is embraced among their constituents2 and these are combined in all possible ways and proportions1 so that the number of combinations is almost endless4 ,the solvent power of the subterranean waters varies for different minerals2 and appears often to be greatest for the rarer species4 ,in other words1 there is a sort of selective action1 whereby many minerals which exist in stratified and eruptive rocks1 so thinly diffused as to entirely escape the most refined observation1 are concentrated in veins in masses of sensible size2 and our lists of known minerals and chemical elements are undoubtedly much longer than they would be if these wonderful storehouses of fine minerals which we call veins had never been explored4 ,as a rule1 the minerals in veins form larger and more perfect crystals than we find in either of the other great classes of rocks4 ,this is simply because the conditions are more favorable for crystallization in veins than in dikes or sedimentary strata4 ,in both dike and stratified rocks1 the growing crystals are surrounded on all sides by solid or semi-solid matter2 and1 being thus hampered1 it is simply impossible that they should become either large or perfect4 ,in the vein1 on the other hand1 there are usually no such obstacles to be overcome2 but the crystals1 starting from the walls of the fissure1 grow toward its centre1 their growing ends projecting into a free space1 where they have freedom to develop their normal forms and to attain a size limited only1 in many cases1 by the breadth of the fissure4 ,with1 possibly1 some rare exceptions1 all the large and perfect crystals of quartz1 feldspar1 mica1 beryl1 apatite1 fluorite1 and of minerals generally1 which we see in mineralogical cabinets1 have originated in veins4 ,those fissures which become the seats of mineral veins are really ,nature's laboratories for the production of rare and beautiful mineral specimens2 and hence the vein rocks are the chief resort of the mineralogist1 to whom they are of far greater interest than all the eruptive and stratified rocks combined4 ,the leading characteristics1 then1 of the vein rocks may be summarized as follows3 7#a7 ,they contain nearly all known minerals1 including many rare species and elements which are unknown outside of this class of rocks4 7#b7 ,these mineral constituents1 occurring singly and combined1 give rise to a number of varieties of rocks so vast as to baffle detailed description4 7#c7 ,they exceed all other rocks in the coarseness of their crystallization1 and in the perfection and beauty of the single crystals which they afford4 ,,petrology4 ,in lithology we investigate the nature of the materials composing the earth's crust--the various minerals and aggregates of minerals1 or rocks2 while in petrology we consider the forms and modes of arrangement of the rock-masses1--in other words1 the architecture of the earth4 ,petrology is the complement of lithology1 and in many respects it is the most fascinating division of geology1 since in no other direction in this science are we brought constantly into such intimate relations with the beautiful and sublime in nature4 ,the structures of rocks are the basis of nearly all natural scenery2 for what we call scenery is usually merely the external expression1 as developed by the powerful but delicate sculpture of the agents of erosion--rain and frost1 rivers and glaciers1 etc4--of the geological structure of the country4 ,and to the practised eye of the geologist1 a fine landscape is not simply a pleasantly or grandly diversified surface1 but it has depth2 for he reads in the superficial lineaments the structure of the rocks out of which they are carved4 ,but1 while the magnitude of the phenomena adds greatly to the charm of the study1 it also increases the difficulties and taxes the ingenuity of the teacher whose work must be done indoors4 ,according to our ideal method1 natural science ought to be taught with natural specimens2 and yet here our main reliance must be upon pictures and diagrams4 ,nature1 however1 has not been wholly unmindful of our needs2 for she has worked often upon a very small as well as a very large scale2 many of the grandest phenomena being repeated in miniature4 ,thus we observe rock-folds or arches miles in breadth and forming mountain masses1 and of all sizes from that down to the minutest wrinkle4 ,so with veins1 faults1 etc4 ,and the wonderful thing is that these small examples1 which may be brought into the class-room1 are usually1 except in size1 exactly like the large4 ,now the aim of every teacher in this department should be to secure a collection of these natural models4 ,it is not an easy thing to do1 except one has plenty of time2 for they can rarely be purchased of dealers1 but must usually come as the choicest fruit of repeated excursions to the natural ledges and quarries1 the seashore and the mountains4 ,but for the difficulty of getting the specimens there is some compensation1 since it may be truly said that for the collector specimens obtained in this way have an interest1 a value1 and a power of instruction beyond what they would otherwise possess4 ,classification of ,structures4 ,the structures of rock divide1 at the outset1 into two classes3--7#a7 the original structures1 or those produced at the same time and by the same forces as the rocks themselves1 and which are1 therefore1 peculiar to the class of rocks in which they occur 7e4g4 stratification1 ripple-marks1 fossils1 etc472 and 7#b7 the subsequent structures1 or those developed in rocks subsequently to their formation1 and by forces that act more or less uniformly upon all classes of rocks1 and which are1 therefore1 in a large degree1 common to all kinds of rocks 7e4g4 folds1 faults1 joints1 etc474 ,the original structures are conveniently and naturally classified in accordance with the three great classes of rocks3 7#a7 stratified rocks1 7#b7 eruptive rocks1 and 7#c7 vein rocks2 while the subsequent structures1 not being peculiar to particular classes of rocks1 are properly divided into those produced by 7#a7 the subterranean or igneous agencies1 and 7#b7 the superficial or aqueous agencies4 ,illustration3 ,fig4 #a4--,section through sediment deposited by rain in a roadside pool3 a4 surface of roadway2 b4 layer of small pebbles and coarse sand2 c4 fine sand passing into d2 d4 the finest sand and mud4 ,original ,structures of ,stratified ,rocks4 ,,stratification4--,all rocks formed by strewing materials in water1 and their deposition in successive1 parallel1 horizontal layers1 are stratified2 and this structure is their stratification4 ,it is the most important of all rock structures2 and there is no kind of structure the origin of which is more fully or certainly known4 ,the deposition of sediment in carefully assorted horizontal layers is readily brought within the comprehension of children by simple experiments with sand and clay in water2 and still better by the examination of the deposits formed in roadside pools during heavy rains 7,fig4 #a71 and by digging into beaches and sandbars1 which every child will recognize as formed of materials arranged by water4 ,great stress should be laid upon the fact that a lake like ,erie or ,champlain is simply a large pool with several more or less turbid streams flowing into it1 while the single stream flowing out is clear1 the sediment having evidently been deposited in the lake2 and that every lake is1 like the roadside pool1 being gradually filled up with sedimentary or stratified rocks4 ,but the ocean is a still larger pool1 receiving mud and sand from many streams2 and since we know that nothing escapes from the ocean but invisible vapor1 it is plain that the mud and sand and all other kinds of sediment carried into the ocean must be deposited on its floor1 and chiefly1 as we have seen1 on that part nearest the land4 ,the consolidation of beaches1 bars1 and mud-flats is all that is necessary to convert them into stratified formations of conglomerate sandstone and slate4 ,let us notice now1 more particularly1 the causes of visible stratification4 ,as we can easily prove by an experiment with clay in a bottle of water1 if the same kind of material is deposited continuously there will be no visible stratification in the deposit4 ,it will be as truly stratified as any formation1 but not visibly so2 because there is nothing in the nature of the material or the way in which it is laid down to bring out distinct lines of stratification4 ,continuous and uniform deposition obtains very frequently in nature1 but rarely continues long enough to permit the formation of thick beds or strata4 ,hence1 while the stratification is almost always visible on the large surfaces of sandstone1 slate1 etc41 exposed in quarries and railway cuttings1 and may usually be seen in the quarried blocks1 it is often not apparent in hand specimens1 which may represent a single homogeneous layer4 ,there is one important exception1 and that is where the particles1 although of the same kind1 are flat or elongated4 ,pebbles of these forms are common on many beaches2 and since they are necessarily arranged horizontally by the action of the water1 they will1 by their parallelism1 make the stratification of the pudding-stone visible4 ,the same result is accomplished still more distinctly by the mica scales1 etc41 in sandstone and slates1 the leaves and flattened stems of vegetation in bituminous coal1 and the flat shells in limestone4 ,in all other cases1 visible stratification implies some change in the conditions2 either the deposition was interrupted1 or different kinds of material were deposited at different times4 ,the first cause produces planes of easy splitting1 or fissility1 especially in fine-grained rocks1 like shale4 ,this shaly structure or lamination-cleavage may be due1 in some cases1 to pressure1 but it is commonly understood to mean that each thin layer of clay became partially consolidated before the next one was deposited upon it1 so that the two could not perfectly cohere4 ,parallel planes of easy splitting are1 however1 by themselves1 of little value as indications of stratification1 since the lamination-cleavage is not easily distinguished from slaty-cleavage 7roofing slate7 and parallel jointing1 structures developed subsequently to the deposition of sediments and quite independent of the stratification4 ,the second cause1 or variations in the kind of sediment1 gives alternating layers differing in color1 texture1 or composition1 as is seen frequently in sandstone1 slate1 gneiss1 etc42 and of all the indications of stratification these are the most important and reliable4 ,illustration3 ,fig4 #b4--,section showing strata and lamin3 a4 conglomerate2 b4 sandstone2 c4 shale2 d4 limestone4 ,a layer composed throughout of essentially the same kind of rock1 as conglomerate or sandstone1 and showing no marked planes of division1 is usually regarded as one bed or stratum1 although it may vary considerably in texture or color2 while the thinner portions composing the stratum and differing slightly in color1 texture1 and composition1 and the thin sheets into which shaly rocks split1 are the lamin or leaves4 ,in ,fig4 #b the strata are designated by letters1 and the fine lines and rows of dots show the constituent lamin1 while the whole section may be regarded as a small part of a great geological formation4 ,the geological record is written chiefly in the sedimentary rocks2 and the formations1 strata1 and lamin may be regarded as the volumes1 chapters1 and pages in the history of the earth4 ,now every feature of a rock1 lithological or petrological1 finds its highest interest in the light which it throws upon the history of the rock1 i4e41 upon the conditions of its formation4 ,observe what the section in ,fig4 #b teaches concerning the geological history of that locality2 premising that any chapter of geological history written in the stratified rocks should be read from the bottom upwards1 since the lowest strata must have been formed first and the highest last4 ,the lowest stratum exposed is conglomerate1 indicating a shingle beach swept by strong currents which carried away the finer material4 ,upwards1 the conglomerate becomes finer and shades off into sandstone1 and finally into shale1 showing that the water has become gradually deeper and more tranquil1 the shore having1 in consequence of the subsidence1 advanced toward the land4 ,the next two strata show that this movement is probably reversed2 at any rate1 the currents become stronger again1 and the shale passes gradually into sandstone and conglomerate4 ,the beach condition prevails now for a long time1 and thick beds of sand and gravel are formed4 ,the sea then deepens again1 and we observe a third passage from coarse to fine sediment4 ,this locality is now remote from the shore1 the gentle currents bringing only the finest mud1 which slowly builds up the thick bed of shale1 in the upper part of which shells are abundant1 indicating that the deposition of mechanical sediment has almost ceased1 and that the shale is changing to limestone4 ,the purity of the limestone1 and the crinoids and other marine organisms which it contains1 prove that this has now become the deep1 clear sea2 and this condition is maintained for a long period1 for the limestone is very thick1 and this rock is formed with extreme slowness4 ,the most important point to be gained here is that every line of stratification and every change in the character of the sediments is due to some change of corresponding magnitude in the conditions under which the rock was formed4 ,the slight and local changes in the conditions occur frequently and mark off the individual lamin and strata1 while the more important and wide-spread changes determine the boundaries of the groups of strata and the formations4 ,strata are subject to constant lateral changes in texture and composition1 i4e41 a bed or formation rarely holds the same lithological characteristics over an extended area4 ,there are some striking exceptions1 especially among the finer-grained rocks1 like slate1 limestone1 and coal1 which have been deposited under uniform conditions over wide areas4 ,it is the general rule1 however1 particularly with the coarse-grained rocks1 which have been deposited in shallow water near the land1 that the same continuous stratum undergoes great changes in thickness and lithological character when followed horizontally4 ,a stratum of conglomerate becomes finer grained and gradually changes into sandstone1 which shades off imperceptibly into slate1 and slate into limestone1 etc4 ,where the stratum is conglomerate1 its thickness will usually be much greater and more variable than where it is composed of the finer sediments4 ,the rapidity of these changes in certain cases is well shown by the parallel sections in ,fig4 #c4 ,these represent precisely the same beds1 as the connecting lines indicate1 at points only twenty feet apart4 ,illustration3 ,fig4 #c4--,parallel sections showing rapid lateral changes in strata3 c4 clay2 s4 sand2 ss4 sandstone2 l4 lignite2 f4 fireclay4 ,when we glance at the conditions under which stratified rocks are now being formed1 it is plain that all strata must terminate at the margin of the sea in which they were deposited1 and in the marginal portions of that sea1 especially1 must exhibit frequent and rapid changes in composition1 etc4 ,the sediments forming the surface of the sea-bottom at the present time may be regarded as belonging to one continuous stratum2 and it is instructive to examine a chart of any part of our coast1 such as ,massachusetts ,bay1 on which the nature of the bottom is indicated for each sounding1 and observe the distribution of the different kinds of sediment4 ,on an irregular coast like this1 especially1 the gravel1 sand1 and mud of different colors and textures1 and the different kinds of shelly bottom1 form a patchwork1 the patches being1 for the most part1 of limited extent and shading off gradually into each other4 ,on a more regular coast1 like that of ,new ,jersey1 the sediments are distributed with corresponding uniformity1 the changes are less frequent and more gradual1 and we have here a better chance to observe the normal arrangement of the sediments along a line from the shore seawards--gravel1 sand1 mud1 and shells4 ,on the beach we find the shingle and coarse pebbles1 shading off rapidly into fine pebbles and sand4 ,the zone or belt of sandy bottom may vary in width from a mile or two to twenty miles or more1 becoming gradually finer and changing into clay or mud1 which covers1 usually1 a much broader zone1 sometimes extending into the deeper parts of the sea1 but gradually giving way to calcareous sediments4 ,hence we may say that the finer the sediment the greater the area over which it is spread2 but1 on the other hand1 the coarser the sediment the more rapidly it increases in thickness4 ,in other words1 the horizontal extent of a formation deposited in any given period of time is inversely1 and the vertical extent or thickness is directly1 proportional to the size of the particles4 ,observations made in deep wells and mines1 and where1 by upturning and erosion1 the edges of the strata are exposed on the surface1 show that the vertical order of the different kinds of sedimentary rocks in the earth's crust is extremely variable4 ,but when we take a general view of a great formation1 it is often apparent that it consists chiefly of coarse-grained rocks in the lower part and fine-grained rocks in the upper part4 ,this is1 in general1 a necessary consequence of the fact that a great thickness of sediments can only be formed on a subsiding sea-floor4 ,such a formation must consist chiefly of shore deposits1 and be deposited near the shore where the sea is shallow4 ,hence1 #aj1jjj feet of sediments implies nearly that amount of subsidence4 ,in consequence1 the shore line and the several zones of sediment advance towards the land2 and sand is deposited where gravel was at first1 and as the subsidence continues1 both clay and limestone are finally deposited over the original beach4 ,when the sea-floor rises1 the order of the sediments is reversed2 and it will be observed that in consequence of the advance and retreat of the shore-line1 the formations grow edgewise to a considerable extent4 ,illustration3 ,fig4 #d4--,overlap and unconformability4 ,,overlap ,,and ,,interposition ,,of ,,strata4--,another consequence of the constant oscillation of the shoreline is that successive deposits in the same sea will often cover different and unequal areas4 ,when1 in consequence of subsidence1 one formation extends beyond and covers the edge of another1 as shown in ,fig4 #d1 we have the phenomenon described as overlap4 ,interposition is similar1 being the case where a formation 7,fig4 #e1 c47 does not1 in certain directions1 cover so wide an area as the strata 7b4 d47 above and below it1 which are thus sometimes found in contact1 although normally separated by the entire thickness of the intermediate and1 seemingly1 interposed stratum4 ,illustration3 ,fig4 #e4--;,interposition of strata4 ,,unconformability4--,we have already seen that the rocks on the land are being constantly worn away by the agents of erosion2 and it is also a matter of common observation that the strata thus exposed are often not horizontal1 but highly inclined1 having been greatly disturbed and crumpled during their elevation4 ,now1 when such a land-surface subsides to form the sea-bottom1 and new strata are spread horizontally over it1 they will lie across the upturned and eroded edges of the older rocks1 and fill the hollows worn out of the latter1 as shown in ,fig4 #f2 and the new formation is then said to rest unconformably upon the older4 ,two strata or formations are unconformable when the older has suffered erosion 7,fig4 #f71 or both disturbance and erosion 7,fig4 #d7 before the deposition of the newer4 ,illustration3 ,fig4 #f4--,unconformability4 ,when strata are conformable1 the deposition may be presumed to have been nearly or quite continuous2 but unconformability clearly proves a prolonged interruption of the deposition during which the elevation1 erosion1 and subsidence of the sea-bottom took place4 ,the section in ,fig4 #g shows a second unconformability1 proving that the sea-bottom has here been lifted three times to form dry land4 ,an unconformability may sometimes be clearly established when the actual contact of the two formations cannot be seen1 as where the new formation is a conglomerate containing fragments of the older4 ,,irregularities ,,of ,,stratification4--,these are especially noticeable in sandstone and conglomerate1 which have been deposited chiefly by strong1 local1 and variable currents2 the kind and quantity of sediment1 of course1 varying with the strength and direction of the current4 ,two kinds of irregularity only may be specially noticed here3 7#a7 contemporaneous erosion and deposit1 where1 in consequence of a change in the currents1 fine material recently deposited is partially swept away and its place taken by coarser sediments2 and 7#b7 oblique lamination1 or current-bedding1 where the strata are horizontal as usual1 but the component lamin are inclined at various angles4 ,this structure is characteristic of sediments swept along by strong currents1 especially when deposited in shallow basins or depressions4 ,illustration3 ,fig4 #g4--;,double ,unconformability3 q4 quartzite2 s4 sandstone2 d4 drift4 ,,ripple-,,marks4--,all who have been on a beach or sand-bar must have noticed the lines of wavy ridges and hollows1 or ripples1 on the surface of the sand4 ,these are sand-waves1 produced by water moving over the sand1 or by air moving over dry sand1 as ordinary waves are formed by air moving over water4 ,each tide usually effaces the ripple-marks made by its predecessor and leaves a new series1 to be obliterated by the next tide4 ,but where sediment is constantly accumulating1 a rippled surface may be gently overspread by a new layer1 and thus preserved4 ,other series of ripples may1 in like manner1 be formed and preserved in overlying layers2 and when the beach becomes a firm sandstone1 a section of it will show the rippled surfaces almost as distinctly as when they were first formed 7,fig4 #h74 ,ripple-marks are most perfect in fine sand4 ,they are not formed in gravel1 because it is too coarse2 nor in clay1 because it is too tenacious4 ,they are usually limited to shallow water2 and are always regarded as proving that the rocks in which they occur are shallow-water or beach deposits4 ,they are normally at right angles to the current that produces them1 and where this changes with the direction of the wind1 cross-ripples and other irregularities are introduced4 ,ripple-marks are also usually parallel with the beach1 and when they are found in the rocks they give us the direction1 as well as the position1 of the ancient shore-line4 ,again1 the friction of the water pushes the sand-grains along1 rolling them up on one side of the ripple and letting them fall down on the other4 ,hence ripples1 formed by a current are always moving and are unsymmetrical on the cross-section1 presenting a long1 gentle slope toward the current1 and a short1 steep slope away from it1 the arrow in the figure indicating the direction of the current1 or of the sea in the case of a beach4 ,and we may thus learn from the fossil ripples1 in some cases1 not only the position and direction of the ancient shore1 but also on which side the land lay1 and on which side the sea4 ,when the water is in a state of oscillation1 without any distinct current1 more symmetrical ripples are produced4 ,illustration3 ,fig4 #h4--,ripple-marks in sandstone4 ,,rill-,,marks1 ,,rain-,,prints1 ,,and ,,sun-,,cracks4--8,one of the most fascinating parts of the work of a field-geologist consists in tracing the shores of former seas and lakes1 and thus reconstructing the geography of successive geological periods40 ,his conclusions1 as we have already seen1 are based largely upon the nature of the sediments2 but still more convincing is the evidence afforded by those superficial features of the strata1 which1 like ripple-marks1 seem1 by themselves1 quite insignificant4 ,and among these he lays special emphasis upon those which show that during their deposition strata have at intervals been laid bare to sun and air4 ,during ebb tide water which has been left at the upper edge of the beach runs down across the beach in small rills1 which excavate miniature channels2 and when these are preserved in the hard rocks1 they prove that the latter are beach deposits1 and1 like the ripple-marks1 show the direction of the old shore4 ,if a heavy shower of rain falls on a muddy beach or flat1 the sediment deposited by the returning tide may cover1 without obliterating1 the small but characteristic impressions of the individual drops2 and these markings are frequently found well preserved in the hardest slates and sandstones1 testifying unequivocally to the conditions under which the rocks were formed4 ,in some cases the rain-prints are found to be ridged up on one side only1 in such a manner as to indicate that the drops as they fell were driven aslant by the wind4 ,the prominent side of the marking1 therefore1 indicates the side towards which the wind blew4 ,muddy sediments1 especially in lakes and rivers1 are often exposed to the air and sun during periods of drouth1 and as they gradually dry up1 polygonal cracks are formed4 ,the sediment of the next layer will fill these sun-cracks2 and when1 as often happens1 it is slightly different from the dessicated layer1 they may still be traced4 ,sun-cracks preserved in this way are very characteristic of argillaceous rocks1 and1 of course1 prove that in early times1 as at the present day1 sediments of this class were exposed by the temporary retreat of the water4 ,the foot-prints or trails of land-animals are often1 as in the sandstones and shales of the ,connecticut ,valley1 associated with1 and of course strongly corroborate1 all these other evidences of shore deposits4 ,from the foot-prints preserved in the rocks we pass naturally to the consideration of the fossil remains of plants and animals found entombed in the strata4 ,,fossils4--,although fossils find their highest interest in the light which they throw upon the succession of life on the globe1 they may also be properly regarded as structural features of stratified rocks2 and any one who has seen the dead shells1 crabs1 fishes1 etc41 on the beach will readily understand how fossils get into the rocks4 ,it is not our province here to study the structure of the fossils themselves1 for that would involve us in a course in paleontology1 a task belonging to the biologist rather than the geologist2 but we will merely observe the three principal degrees in the preservation of fossils3-- #a4 ,original composition not completely changed4--,extinct elephants have been found frozen in the river-bluffs of ,siberia so perfectly preserved that dogs and wolves ate their flesh4 ,the bodies of animals are also found well preserved in peat-bogs4 ,all coal is simply fossil vegetation retaining in a large degree the original composition2 and the same is true of ferns1 etc41 preserved as black impressions in the rocks4 ,all bones and shells consist of mineral matter which makes them hard1 and animal matter which makes them tough and strong4 ,in very many cases1 especially in the newer formations1 the animal matter is still partially1 and the mineral matter almost wholly1 intact4 #b4 ,original composition completely changed1 but form and structure preserved4--,all kinds of fossils are commonly called petrifactions1 but only those preserved in this second way are truly petrified1 i4e41 turned to stone4 8,petrified wood is the best illustration1 and in a good specimen not only the external form of the wood1 not only its general structure--bark1 wood1 radiating silver-grain1 and concentric rings of growth--are discernible1 but even the microscopic cellular structure of the wood1 and the exquisite sculpturing of the cell-walls1 are perfectly preserved1 so that the kind of wood may often be determined by the microscope with the utmost certainty4 ,yet not one particle of the organic matter of the wood remains4 ,it has been entirely replaced by mineral matter2 usually by some form of silica4 ,the same is true of the shells and bones of animals40--,,le ,,conte4 #c4 ,original composition and structure both obliterated1 and form alone preserved4--,this occurs most commonly with shells1 although fossil trees are also often good illustrations4 ,the general result is accomplished in several ways3 7a7 ,the shell after being buried in the sediment may be removed by solution1 leaving a mould of its external form1 7b7 ,this mould may subsequently be filled by the infiltration of finer sediment1 forming a cast of the exterior of the shell4 7c7 ,the shell1 before its solution1 may have been filled with mud2 and if the shell itself is then dissolved1 we have a cast of its interior in a mould of its exterior4 ,,time ,,required ,,for ,,the ,,formation ,,of ,,stratified ,,rocks4--,many attempts have been made to determine the time required for the deposition of any given thickness of stratified rocks4 ,of course1 only roughly approximate results can be hoped for in most cases2 but these are at least sufficient to make it certain that geological time is very long4 ,the average relative rate of growth of different kinds of sediment is1 however1 less open to doubt1 for we have already seen that coarse sediments like gravel and sand accumulate much more rapidly than finer sediments like clay and limestone2 and we are sometimes able to compare these two classes of rocks on a very large scale4 ,thus1 during what is known as the ,paleozoic era1 a sea extended from the ,blue ,ridge to the ,rocky ,mountains4 ,along the eastern margin of this sea1 where the ,alleghany ,mountains now stand1 sediments--chiefly conglomerate and sandstone1 with some slate and less limestone--accumulated to a thickness of nearly #dj1jjj feet4 ,toward the west1 away from the old shore-line1 the coarse sediments gradually die out1 and the formations become finer and thinner4 ,in western ,ohio and ,indiana1 slate and limestone predominate2 while in the central part of the ancient sea1 in ,illinois and ,missouri1 the paleozoic sediments are almost wholly limestones1 and have a thickness of only #djjj to #ejjj feet4 ,in other words1 while one foot of limestone was forming in the ,mississippi ,valley1 eight to ten feet of coarser sediments were deposited in ,pennsylvania4 ,the best estimates show that coral-reefs rise--i4e41 limestones are formed on them--at the rate of about one foot in two hundred years4 ,but coral limestones grow much more rapidly than limestones in general4 ,sandstones sometimes accumulate so rapidly that trees are buried before they have time to decay and fall 7,fig4 #i74 ,such a buried forest1 like a coal-bed1 represents a land surface1 and proves a subsidence of the land2 and in some cases1 as indicated by the section1 repeated oscillations of the crust may be proved in this way4 ,the mud deposited by the annual overflow of the ,nile is forty feet thick near the ancient city of ,memphis2 and the pedestal of the statue of ,rameses ,,ii41 believed to have been erected ,b4,c4 #acfa1 is buried to a depth of nine feet1 four inches1 indicating that #ac1ejj years have elapsed since the ,nile began to spread its mud over the sands of the desert4 ,illustration3 ,fig4 #i4--;,erect fossil trees4 ,but the greatest difficulty in estimating the time required for the formation of any series of strata arises from the fact that we cannot usually even guess at the length of the periods when the deposition has been partially or wholly interrupted4 ,now and then1 however1 we find evidence that these periods may be very long4 ,a layer of fossil shells in sandstone or slate proves an interruption of mechanical deposition4 ,beds of coal1 fossil forests1 and other indications of land surfaces are still more conclusive4 ,the interposition of strata 7,fig4 #e7 proves a prolonged interruption of deposition over the area not covered by the interposed bed4 ,but the most important of all evidence is that afforded by unconformability2 and the length of the lost interval between the two formations is measured approximately by the erosion of the older4 ,original ,structures of ,eruptive ,rocks4 ,the structures of this class are divisible into those pertaining to the volcanic rocks and those pertaining to the fissure or dike rocks4 ,but since volcanoes are rare in this part of the world1 while dikes are well developed in many sections of our country1 it seems best to give our attention chiefly to the latter4 ,illustration3 ,fig4 #aj4--,typical dikes4 ,illustration3 ,fig4 #aa4--,section of a granite mass4 ,the term dike is a general name for all masses of eruptive rocks that have cooled and solidified in fissures or cavities in the earth's crust4 ,but the name is commonly restricted to the more regular1 wall-like masses 7,fig4 #aj71 those having extremely irregular outlines1 like most masses of granite 7,fig4 #aa71 being known simply as eruptive masses4 ,the propriety of this distinction is apparent when we consider the origin of dike as a geological term4 ,it was first used in this sense in southern ,scotland1 where almost any kind of a wall or barrier is called a dike4 ,the dikes traverse the different stratified formations like gigantic walls1 which are often encountered by the coal-miners1 and on the surface are frequently left in relief by the erosion of the softer enclosing rock1 so that in the west of ,scotland1 especially1 they are actually made use of for enclosures4 ,in other cases the dike has decayed faster than the enclosing rock1 and its position is marked by a ditch-like depression4 ,the narrow1 straight1 and perpendicular clefts or chasms observed on many coasts are usually due to the removal of the wall-like dikes by the action of the waves4 ,dikes are sometimes mere plates of rock1 traceable for a few yards only2 and they range in size from that up to those a hundred feet or more in width1 and traceable for scores of miles across the country1 their outcrops forming prominent ridges4 ,the sides of dikes are often as parallel and straight of those of built walls1 the resemblance to human workmanship being heightened by the numerous joints which1 intersecting each other along the face of a dike1 remind us of well-fitted masonry4 ,,forms ,,of ,,dikes4--,a dike is essentially a casting4 ,melted rock is forced up from the heated interior into a cavity or crack in the earth's crust1 cools and solidifies there1 and1 like a metallic casting1 assumes the form of the fissure or mould4 ,in other words1 the form of the dike is exactly that of the fissure into which the lava was injected4 ,now the forms of fissures depend partly upon the nature of the force that produces them1 but very largely upon the structure--and especially the joint-structure--of the enclosing rocks4 ,nearly all rocks are traversed by planes of division or cracks called joints1 which usually run in several directions1 dividing the rock into blocks4 ,and it is probable that dike-fissures are most commonly produced1 not by breaking the rocks anew1 but by widening or opening the pre-existing joint-cracks4 ,hence the straight and regular jointing of slate1 limestone and most sedimentary rocks is accompanied by wall-like dikes--the typical dikes 7,fig4 #aj72 while the more irregular jointing of granite and other massive rocks gives rise to sinuous1 branching1 variable dikes4 ,the general dependence of dikes upon the joint-structure of the rocks is proved by the facts that dikes1 like joints1 are normally vertical or highly inclined1 and that they are usually parallel with the principal systems of joints in the same district4 ,the wall-like dikes also give off branches1 but usually in a regular manner1 as shown in ,fig4 #ab4 ,illustration3 ,fig4 #ab4--;,dike with regular branches4 ,,structure ,,of ,,dikes4--,the rock traversed by a dike is called the country or wall rock4 ,fragments of this are often torn off by the igneous material1 and become enclosed in the latter4 ,such enclosed fragments may sometimes form the main part of the dike1 which then1 since they are necessarily angular1 often assumes the aspect of a breccia4 ,this is the only important exception to the rule that dikes are homogeneous in composition2 i4e41 in the same dike we can usually find--from end to end1 from side to side1 and probably from top to bottom--no essential difference in composition4 ,but there is often a marked contrast in texture between different parts of a dike1 and especially between the sides and central portion4 ,the liquid rock loses heat most rapidly where it is in contact with the cold walls of the fissure1 and solidifies before it has time to crystallize1 remaining compact and sometimes even glassy2 while in the middle of the dike1 unless it is very narrow1 it cools so slowly as to develop a distinctly crystalline texture4 ,there is no abrupt change in texture1 but a gradual passage from the compact border to the coarsely crystalline or porphyritic middle portion4 ,it is obvious that a similar gradation in texture must exist between the top and bottom of a dike4 ,,contact ,,phenomena4--,under this head are grouped the interesting and important phenomena observable along the contact between the dike and wall-rock4 ,these throw light upon the conditions of formation of dikes1 and are often depended upon to show whether a rock mass is a dike or not4 ,the student will observe here3-- #a4 ,the detailed form of the contact4 ,it may be straight and simple1 or exceedingly irregular1 the dike penetrating the wall1 and enclosing fragments of it1 as in ,fig4 #aa1 which is a typically igneous contact4 #b4 ,the alteration of the wall-rock by heat4 ,this may consist in3 7a7 coloration1 shales and sandstones being reddened in the same way as when clay is burnt for bricks2 7b7 baking and induration1 sandstone being converted into quartzite and even jasper2 clay1 slate1 etc41 being not only baked to a flinty hardness1 but actually vitrified1 as in porcelainite2 and bituminous coal being converted into natural coke or anthracite2 and 7c7 crystallization1 chalk1 and other limestones being changed to marble1 and crystals of pyrite1 calcite1 quartz1 etc41 being developed in slate1 sandstone1 and other rocks4 #c4 ,the alteration of the dike-rock by 7a7 more rapid cooling1 and 7b7 the access of thermal waters4 ,the alteration of the wall-rock may extend only a few inches or many yards from the dike1 gradually diminishing with the distance2 and the cases are surprisingly numerous where there is no perceptible alteration2 and1 again1 the alteration is usually mutual1 the dike-rock being altered in texture1 color1 and composition4 ,illustration3 ,fig4 #ac4--;,ideal cross-section of a laccolite4 ,illustration3 ,fig4 #ad4--;,ideal cross-section of a volcano4 ,,intrusive ,,beds4--,we commonly think of dikes as cutting across the strata1 but they often lie in planes parallel with them2 and the same dike may run across the beds in some parts of its course and between them in others 7,fig4 #ab71 or the conformable dike maybe simply a lateral branch of a main vertical dike1 as shown in the same figure4 ,all dikes or portions of dikes lying conformably between the strata are called intrusive beds or sheets4 ,when a dike fails to reach the surface1 but spreads out horizontally between the strata1 forming a thick dome or oven-shaped intrusive bed1 the latter is called a laccolite 7,fig4 #ac74 ,laccolites are sometimes of immense volume1 containing several cubic miles of rock4 ,fig4 #ad enables us to compare the laccolite with the volcano4 ,in the one case a large mound of eruptive material accumulates between the strata1 the overlying beds being lifted into a dome2 while in the other case the fissure or vent reaches the surface1 and the mound of lava is built up on top of the ground4 ,,cotemporaneous ,,beds4--,when the lava emitted by a crater is sufficiently liquid1 it spreads out horizontally1 forming a volcanic sheet or bed4 ,if such an eruption is submarine1 or the lava flow is subsequently covered by the sea1 sedimentary deposits are formed over it2 and beds of lava which thus come to lie conformably between sedimentary strata are known as cotemporaneous sheets or beds1 because they belong1 in order of time1 in the position in which we find them1 being1 like any member of a stratified series1 newer than the underlying and older than the overlying strata4 ,cotemporaneous lava-flows are sometimes repeated again and again in the same district1 and thus important formations are built up of alternating igneous and aqueous deposits4 ,evidently1 the student who would read correctly the record of igneous activity in the past must be able to distinguish intrusive and cotemporaneous beds4 ,the principal points to be considered in making this distinction are3 7#a7 ,the intrusive bed is essentially a dike1 dense and more or less crystalline in texture1 altering1 and often enclosing fragments of1 both the underlying and overlying strata1 and frequently jogging across or penetrating the sediments4 7#b7 ,the cotemporaneous bed1 on the other hand1 being essentially a lava-flow1 is much less dense and crystalline1 being usually distinctly scoriaceous or amygdaloidal1 especially at the borders1 and the underlying strata alone showing heat action1 or occurring as enclosures in the lava2 for the overlying strata are newer than the lava1 and often consist largely1 at the base1 of water-worn fragments of the lava4 ,,ages ,,of ,,dikes4--,the ages of dikes may be estimated in several ways4 ,they are necessarily newer than any stratified formation which they intersect or of which they enclose fragments2 but any formation crossing the top of a dike must usually be regarded as newer than the dike1 especially if it contains water-worn fragments of the dike rock4 ,the relative ages of different dikes are determined by their relations to the stratified formations2 and still more easily by their mutual intersections1 on the principle that when two dikes cross each other1 the intersecting must be newer than the intersected dike4 ,it is sometimes possible1 in this way1 to prove several distinct periods of eruption in the same limited district4 ,the textures of dikes also often afford reliable indications of their ages2 for1 as we have already seen1 the upper part of a dike1 cooling rapidly and under little pressure1 must be less dense and crystalline than the deep-seated portion1 which cools slowly and under great pressure4 ,now1 the lower1 coarsely crystalline part of a dike can usually be exposed on the surface only as the result of enormous erosion2 and erosion is a slow process1 requiring vast periods of time4 ,hence1 when we see a coarse-grained dike outcropping on the surface1 we are justified in regarding it as very old1 for all the fine-grained upper part has been gradually worn away by the action of the rain1 frost1 etc4 ,other things being equal1 coarse-grained must be older than fine-grained dikes2 and the texture of a dike is at once a measure of its age and of the amount of erosion which the region has suffered since it was formed4 ,,eruptive ,,masses4--,in striking contrast with the more or less wall-like dikes are the highly irregular1 and even ragged1 outlines of the eruptive masses2 and it is worth while to notice the probable cause of this contrast4 ,the true dikes are formed1 for the most part1 of comparatively fine-grained rocks--the typical 8traps02 while the eruptive masses consist chiefly of the coarse-grained or granitic varieties4 ,now we have just seen that the coarse-grained rocks have been formed at great depths in the earth's crust1 while the fine-grained are comparatively superficial4 ,but we have good reason for believing that the joint-structure1 upon which the forms of dikes so largely depend1 is not well developed at great depths1 where the rocks are toughened1 if not softened1 by the high temperature4 ,in other words1 trap dikes are formed in the jointed formations1 which break regularly2 while the granitic masses are formed where the absence of joint-structure and a high temperature combine to cause extremely irregular rifts and cavities when the crust is broken4 ,,volcanic ,,pipes ,,or ,,necks4--,every volcano and every lava-flow or volcanic sheet must be connected with the earth's interior by a channel or fissure1 which becomes a dike when the lava ceases to flow4 ,but the converse proposition is not true1 for it is probable that many dikes did not originally reach the surface1 but have been exposed by subsequent denudation4 ,this is conspicuously the case with laccolites and other forms of intrusive sheets4 ,volcanic sheets or beds have probably often resulted from the overflow of the lava at all points of an extensive fissure or system of fissures2 but the vent of the true volcano must be more circumscribed1 an approximately circular opening in the earth's crust1 although doubtless originating in a fissure or at the intersection of two or more fissures1 the lava continuing to flow at the widest part of the wound in the crust long after it has congealed in the narrower parts4 ,such a tube is known as the neck or pipe of the volcano2 and volcanic necks are a highly interesting class of dikes1 since they determine the exact location of many an ancient volcano1 where the volcanic pile itself has long since been swept away4 ,necks and dikes are the downward prolongations or roots of the volcanic cone or sheet1 and cannot be exposed on the surface until the volcanic fires have gone out and the agents of erosion have removed the greater part of the ejected materials4 ,hence1 equally with the dikes which originally failed to reach the surface1 they1 wherever open to our observation1 testify to extensive erosion and a vast antiquity4 ,original ,structures of ,vein ,rocks4 ,many things called veins are improperly so called1 such as dikes of granite and trap1 and beds of coal and iron-ore4 ,the smaller1 more irregular1 branching dikes1 especially1 are very commonly called veins1 and to distinguish the true veins from these eruptive masses1 they are designated as mineral veins or lodes1 although the term lode is usually restricted to the metalliferous veins4 ,,origin ,,of ,,veins4--,various theories of the formation of veins have been proposed1 but the most of these are of historic interest merely1 for geologists are now well agreed that nearly all true veins have been formed by the deposition of minerals from solution in fissures or cavities in the earth's crust4 ,in many cases1 especially where the veins are of limited extent1 it seems probable that a part or all of the mineral matter was derived from the immediately enclosing rocks1 being dissolved out by percolating water2 and these are known as segregation or lateral secretion veins4 ,but it is quite certain that as a general rule the mineral solutions have come chiefly from below1 the deep-seated thermal waters welling up through any channel opened to them1 and gradually depositing the dissolved minerals on the walls of the fissure as the temperature and pressure are diminished4 ,this case1 however1 differs from the first only in deriving the vein-forming minerals from more remote and deeper portions of the enclosing rocks2 and thus we see that vein-formation1 whether on a large or a small scale1 is always essentially a process of segregation4 ,we know that every volcano and every lava flow must be connected below the surface with a dike2 and it is almost equally certain that the waters of mineral springs forming tufaceous mineral deposits on the surface1 as in the geyser districts1 also deposit a portion of the dissolved minerals on the walls of the subterranean channels1 which are thus being gradually filled up and converted into mineral veins1 which will be exposed on the surface when erosion has removed the tufaceous overflow4 ,this connection of vein-formation with the superficial deposits of existing springs has been clearly proved in several important instances in ,nevada and ,california4 ,veins occur chiefly in old1 metamorphic1 and highly disturbed formations1 where there is abundant evidence of the former existence of profound fissures1 and in regions similar to those in which thermal springs occur to-day4 ,in the supplement to the lithological section the student will find the formation of a typical vein briefly described and contrasted with that of a typical dike2 also a brief account of the lithological peculiarities of vein rocks1 and general statements concerning their relative abundance and vast economic importance4 ,,external ,,characteristics ,,of ,,veins4--,the typical vein may be described as a fissure of indefinite length and depth1 filled with mineral substances deposited from solution4 ,externally1 it is very similar to the typical dike1 for the fissures are made in the same way for both4 ,veins are normally highly inclined to the horizon2 they exhibit in nearly every respect the same general relations to the structure of the country rock as dikes2 and the ages of veins are determined in the same way as the ages of dikes4 ,the extensive mining operations to which veins have been subjected in all parts of the world1 have made our knowledge of their forms below the surface very full and accurate4 ,it has been learned in this way that very often the corresponding portions of the walls of a vein do not coincide in position1 but one side is higher or lower than the other1 showing that the walls slipped over each other when the fissure was formed or subsequently2 and this faulting or displacement of the walls appears to be much more common with veins than with dikes1 perhaps because the fissures remained open much longer4 ,this slipping of the walls is the principal cause of the almost constant changes in the width of veins4 ,for1 since the walls are never true planes1 and are often highly irregular any unequal movements must bring them nearer together at some points than at others4 ,as a rule1 the enormous friction accompanying the faulting1 either crushes the wall-rock1 or polishes and striates it1 producing the highly characteristic surfaces known as slicken-sides4 ,where the wall is finely pulverized in this way1 or is partially decomposed before or after the filling of the fissure1 a thin layer of soft1 argillaceous material is formed1 separating the vein proper from the wall-rock4 ,the miners call this the selvage2 and it is a very characteristic feature of the true fissure veins4 ,fragments of the wall-rock are frequently enclosed in veins1 and the latter sometimes branch or divide in such a way as to surround a large mass of the wall1 which is known as a 8horse40 ,a similar result is accomplished when a fissure is re-opened after being filled1 if the new fissure does not coincide exactly with the old4 ,it has been proved that veins have thus been re-opened and filled several times in succession2 and in this way fragments of the older vein material become enclosed in the newer4 ,although usually determined in direction by the joint-structure of the country rock1 veins are often parallel with the bedding1 especially in highly inclined1 schistose formations4 ,such interbedded veins are commonly distinctly lenticular in form1 occupying rifts in the strata which thin out in all directions and are often very limited in extent4 ,whether conforming with the joint-structure or bedding1 veins are commonly arranged in systems by their parallelism1 those of different systems or directions usually differing in age and composition1 and the older veins being generally faulted or displaced when intersected by the newer4 ,,internal ,,characteristics ,,of ,,veins4--,internally1 veins and dikes are strongly contrasted2 and it is upon the internal features1 chiefly1 as previously explained1 that we must depend for their distinction4 ,in metalliferous veins the minerals containing the metal sought for 7the galenite1 sphalerite1 etc47 are the ore2 while the non-metalliferous minerals 7the quartz1 feldspar1 calcite1 etc47 are called the gangue or vein-stone proper4 ,although the combinations of minerals in veins are almost endless1 yet certain associations of ores with each other and with different gangue minerals are tolerably constant1 and constitute an important subject for the student of metallurgy and mining4 ,when a vein is composed of a single mineral1 as quartz1 it may rival a dike in its homogeneity4 ,most important veins1 however1 are composed of several or a large number of minerals1 which may be sometimes more or less uniformly mixed with each other1 but are usually distributed in the fissure in a very irregular manner4 ,the great granite veins which are worked for mica1 feldspar and quartz1 are good illustrations1 on a large scale1 of the structure of veins in which several minerals have been deposited cotemporaneously4 ,the individual minerals are found to a large extent1 in great1 irregular masses1 with no order observable in their arrangement4 ,when a mineral is deposited from solution1 it crystallizes by preference on a surface of similar composition1 thus quartz on quartz1 feldspar on feldspar1 and so on2 and it seems probable that this selective action of the wall-rock may be a principal cause of the irregular distribution of minerals in veins4 ,it has often been observed in metalliferous veins that the richness varies with the nature of the adjacent country rock4 ,this dependence of the contents of a fissure upon the wall-rock may be due in part to the selective deposition of the minerals1 and in part to their derivation from the contiguous portions of the country or wall-rock1 as in the so-called segregated veins4 ,temperature and pressure exert an important influence upon chemical precipitation1 and it is1 therefore1 probable that the composition of many veins varies with the depth4 ,illustration3 ,fig4 #ae4--;,ideal section of a vein4 ,frequently1 perhaps usually1 the minerals of composite veins are deposited in succession1 instead of cotemporaneously1 giving rise to the remarkable banded structure so characteristic of this class of veins4 ,the first mineral deposited in the fissure forms a layer covering each wall1 and is in turn covered by layers of the second mineral1 and that by the third1 and so on1 until the fissure is filled1 or the solution exhausted4 ,the distinguishing features of this structure are shown in ,fig4 #ae1 in which w w represents the wall-rock1 a a1 b b1 c c are successive layers of quartz1 fluorite and barite1 and the central band1 d1 is galenite4 ,since the vein grows from the outside inward1 the outer layers are the oldest1 and the central layers are the newest2 again1 the layers are symmetrically arranged1 being repeated in the reverse order on opposite sides of the middle of the vein2 and1 lastly1 in layers composed of prismatic crystals1 as quartz 7see the figure72 the crystals are perpendicular to the wall and often project into1 and even through1 the succeeding layers4 ,such a crystalline layer is called a 8comb0 and the interlocking of the layers in this way is described as the comb-structure of the vein4 ,the banding of veins is thus strongly contrasted with stratification1 and with the structure in dikes due to the more rapid cooling along the walls4 ,the duplicate layers are often discontinuous and of unequal thickness1 on account of the strong tendency to segregation in the materials4 ,this is clearly shown in ,fig4 #af1 drawn on a reduced scale from a polished section of a lead vein in ,cumberland1 ,england1 contained in the ,museum of the ,boston ,society of ,natural ,history4 ,in this the gangue minerals are fluorite 7f7 and barite 7b74 ,the central band 7f g7 is a darker fluorite containing irregular masses of galenite4 ,the banded structure of veins is exactly reproduced in miniature in the banding of agates1 geodes1 and the amygdules formed in old lavas4 ,unfilled cavities frequently remain along the middle of the vein4 ,when small1 these are known as 8pockets40 ,they are commonly lined with crystals2 and when the latter are minute1 the pockets are called druses4 ,in metalliferous veins1 the ore is much more abundant in some parts than in others1 and these ore-bodies1 especially when somewhat definite in outline1 are known in their different forms and in different localities1 as courses1 slants1 shoots1 chimneys1 and bonanzas of ore4 ,the intersections and junctions of veins are often among the richest parts1 as if the meeting of dissimilar solutions had determined the precipitation of the ore4 ,illustration3 ,fig4 #af4--,section of a lead vein1 one-fifth natural size4 ,metalliferous veins1 especially1 are usually deeply decomposed along the outcrop by the action of atmospheric agencies4 ,the ore is oxidized1 and to a large extent removed by solution1 leaving the quartz and other gangue minerals in a porous state1 stained by oxides of iron1 copper1 and other metals1 forming the gossan or blossom-rock of the vein4 ,,peculiar ,,types ,,of ,,veins4--,in calcareous or limestone formations1 especially1 the joint-cracks and bedding-cracks are often widened through the solution of the rock by infiltrating water1 and thus become the channels of a more or less extensive subterranean drainage1 by which they are more rapidly enlarged to a system of galleries and chambers1 and1 in some cases1 large limestone caverns4 ,the water dripping into the cavern from the overlying limestone is highly charged with carbonate of lime1 which is largely deposited on the ceiling and floor of the cavern1 forming stalactitic and stalagmitic deposits4 ,these are masses of mineral matter deposited from solution in cavities in the earth's crust1 and are essentially vein-formations4 ,portions of caverns deserted by the flowing streams by which they were excavated1 are often filled up in this way1 being converted into irregular veins of calcite4 ,but calcite is not the only mineral found in these cavern deposits1 for barite and fluorite1 and various lead and zinc ores1 especially the sulphides of these metals--galenite and sphalerite--have also been leached out of the surrounding limestone and concentrated in the caverns4 ,the celebrated lead mines of the ,mississippi ,valley1 and some of the richest silver-lead mines of ,utah and ,nevada are of this character4 ,the forms of these cavern-deposits vary almost indefinitely1 and are often highly irregular4 ,the principal types are known as gash-veins1 flats and sheets 7,fig4 #ag71 chambers and pockets4 ,where joints and other cracks have opened slightly in different directions and become filled with infiltrated ores1 we have what the ,german miners call a stock-work1--an irregular network of small and interlacing veins4 ,illustration3 ,fig4 #ag4--;,gash-veins and sheets4 ,an impregnation is an irregular segregation of metalliferous minerals in the mass of some eruptive or crystalline rock4 ,its outlines are not sharply defined1 but it shades off gradually into the enclosing rock4 ,fahlbands are similar ill-defined deposits or segregations in stratified rocks4 ,an impregnation or vein occurring along the contact between two dissimilar rocks is called a contact deposit4 ,these are usually found between formations of different geological ages1 and especially between eruptive and sedimentary rocks4 ,subsequent ,structures produced by ,subterranean ,agencies4 ,the subterranean forces concerned in the formation of rocks are chiefly various manifestations of that enormous tangential pressure developed in the earth's crust1 partly by the cooling and shrinkage of its interior1 but largely1 it is probable1 by the diminution of the velocity of the earth's rotation by tidal friction1 and the consequent diminution of the oblateness of its form4 ,it is well known that the centrifugal force arising from the earth's rotation is sufficient to change the otherwise spherical form of the earth to an oblate spheroid1 with a difference of twenty-six miles between the equatorial and polar diameters4 ,it is also well known that while the earth turns from west to east on its axis1 the tidal wave moves around the globe from east to west1 thus acting like a powerful friction-brake to stop the earth's rotation4 ,our day is consequently lengthening1 and the earth's form as gradually approaching the perfect sphere4 ,this means a very decided shortening and consequent crumpling of the equatorial circumference1 and is equivalent to a marked shrinkage of the earth's interior1 so far as the equatorial regions are concerned4 ,the most important and direct result of the horizontal thrust1 whether due to cooling or tidal friction1 is the corrugation or wrinkling of the crust2 and the earth-wrinkles are of three orders of magnitude1--continents1 mountain-ranges1 and rock-folds or arches4 ,continents and ocean-basins1 although the most important and permanent structural features of the earth's crust1 do not demand further consideration here1 since their forms and relations are adequately described in the better text-books of physical geography4 ,the forms and distribution of mountain-ranges might be dismissed in the same way2 but1 unlike continents1 the structure of mountains1 upon which their reliefs mainly depend1 is quite fully exposed to our observation1 and is one of the most important fields of the student of structural geology4 ,mountains1 however1 as previously explained1 combine nearly all the kinds of structure produced by the subterranean agencies1 and their consideration1 therefore1 belongs at the end rather than the beginning of this section4 ,,inclined ,,or ,,folded ,,strata4--,normally1 strata are horizontal1 and dikes and veins are vertical or nearly so4 ,hence the stratified rocks are more exposed to the crumpling action of the tangential pressure in the earth's crust than the eruptive and vein rocks2 and it is for this reason and partly because the stratified rocks are vastly more abundant than the other kinds1 that the effects of the corrugation of the crust are studied chiefly in the former4 ,but it should be understood that folded dikes and veins are not uncommon4 ,that the stratified rocks have1 in many instances1 suffered great disturbance subsequent to their deposition1 is very evident2 for1 while the strata must have been originally approximately straight and horizontal1 they are now often curved1 or sharply bent and contorted1 and highly inclined or even vertical4 ,all inclined beds or strata are portions of great folds or arches4 ,thus we may feel sure when we see a stratum sloping downward into the ground1 that its inclination or dip does not continue at the same angle1 but that at some moderate depth it gradually changes and the bed rises to the surface again4 ,similarly1 if we look in the opposite direction and think of the bed as sloping upward--we know that the surface of the ground is being constantly lowered by erosion1 and consequently that the inclined stratum formerly extended higher than it does now1 but not indefinitely higher2 for1 in imagination1 we see it curving and descending to the level of the present surface again4 ,hence it forms1 at the same time1 part of one side of a great concave arch1 and of a great convex arch1 just as every inclined surface on the ground indicates both a hill and a valley4 ,and guided by this principle we can often reconstruct with much probability folds that have been more or less completely swept away by erosion1 or that are buried beyond our sight in the earth's crust4 ,the arches of the strata are rarely distinctly indicated in the topography1 but must be studied where the ground has been partly dissected1 as in cliffs1 gorges1 quarries1 etc4 ,they are also1 as a rule1 far more irregular and complex than they are usually conceived or represented4 ,the wrinkles of our clothing are often better illustrations of rock-folds than the models and diagrams used for that purpose4 ,this becomes self-evident when we reflect that the earth's crust is exceedingly heterogeneous in composition and structure1 and must1 therefore1 yield unequally to the unequal strains imposed upon it4 ,the folds or undulations of the strata may be profitably compared with water-waves4 ,in fact1 the comparison is so close that they have been not inaptly called rock-waves4 ,folds1 like waves1 unless very large1 rarely continue for any great distance1 but die out and are replaced by others1 giving rise to the en echelon or step-like arrangement4 ,the plan of both a wave and a fold is a more or less elongated ellipse1 each stratum in a fold being semi-ellipsoidal or boat-shaped4 ,in other words1 a normal fold is an elongated mound of concentric strata1 being highest at the centre1 sloping very gradually toward the ends1 and much more abruptly toward the sides4 ,illustration3 ,fig4 #ah4--;,anticlinal and synclinal folds4 ,the imaginary line passing longitudinally through a fold1 about which the strata appear to be bent1 is the axis2 and the plane lying midway between the two sides of a fold and including the axis is the axial plane4 ,the two principal kinds of folds are the anticline 7,fig4 #ah1 ,a71 where the strata dip away from the axis2 and the syncline 7,fig4 #ah1 ,b71 where they dip toward the axis4 ,they are commonly1 but not always1 correlative1 like hill and valley4 ,rock-folds are of all sizes1 from almost microscopic wrinkles to great arches miles in length and breadth1 and thousands of feet in height4 ,the smaller folds1 or such as may be seen in hand specimens and even in considerable blocks of stone1 are commonly called contortions1 and it is interesting to observe that they are1 in nearly everything except size1 precisely like the large folds1 so that they answer admirably as geological models4 ,large folds1 however1 are almost necessarily curves1 but contortions are frequently angular 7,fig4 #ai74 ,with folds1 as with waves1 the small undulations are borne upon the large ones2 but the contortions are not uniformly distributed4 ,an inspection of ,fig4 #ah shows that when the rocks are folded they must be in a state of tension on the anticlines 7,a71 and in a state of compression in the synclines 7,b71 and the latter is evidently the normal position of the puckerings or contortions of the strata1 as shown in ,fig4 #bj4 ,contortions are also most commonly found in thin-bedded1 flexible rocks1 such as shales and schists4 ,and when we find them in hard1 rigid rocks1 like gneiss and limestone1 it must mean either that the structure was developed with extreme slowness1 or that the rock was more flexible then and possibly plastic4 ,illustration3 ,fig4 #ai4--;,contorted strata4 ,illustration3 ,fig4 #bj4--;,contorted syncline4 ,illustration3 ,fig4 #ba4--,section of anticlinal mountains4 ,it is very interesting to notice the relations of anticlinal and synclinal folds to the agents of erosion4 ,at the time the folds are made1 the anticlinals1 of course1 are ridges1 and the synclinals1 valleys1 and this relation sometimes continues1 as shown in ,fig4 #ba2 but we have seen that the rocks in the trough of the synclinal are compressed and compacted1 i4e41 made more capable of resisting erosion1 while those on the crest of the anticlinal are stretched and broken1 i4e41 made more susceptible of erosion4 ,the consequence is that the anticlinals are usually worn away very much faster than the synclinals2 so much faster that in many cases the topographic features are completely transposed1 and in place of anticlinal ridges and synclinal valleys 7,fig4 #ba7 we find synclinal ridges and anticlinal valleys 7,fig4 #bb74 ,illustration3 ,fig4 #bb4--,section of synclinal mountains4 ,illustration3 ,fig4 #bc4--,monoclinal fold4 ,illustration3 ,fig4 #bd4--,unsymmetrical and inverted folds4 ,besides the anticlinal and synclinal folds already explained1 there are folds that slope in only one direction1 one-sided or monoclinal folds 7,fig4 #bc74 ,anticlinal and synclinal folds are symmetrical when the dip or slope of the strata is the same on both sides and the axial plane is vertical4 ,the great majority of folds1 however1 are unsymmetrical1 the opposite slopes being unequal1 and the axial planes inclined to the vertical 7,fig4 #bd1 ,a74 ,this means that the compressing or plicating force has been greater from one side than from the other1 as indicated by the arrows4 ,it acted with the greatest intensity on the side of the gentler slope1 the tendency evidently having been to crowd or tip the fold over in the direction of the steep slope4 ,when the steep slope approaches the vertical1 this tendency is almost unresisted1 and when it passes the vertical1 gravitation assists in overturning the fold 7,fig4 #bd1 ,b74 ,such highly unsymmetrical folds1 including all cases where the two sides of the fold slope in the same direction1 are described as overturned or inverted1 although the latter term is not strictly applicable to the entire fold1 but only to the strata composing the under or lee side of it4 ,fig4 #bd1 ,b1 shows that these beds are completely inverted1 the older1 as the figures indicate1 lying conformably upon the newer4 ,this inversion is one of the most important features of folded strata1 and it has led to many mistakes in determining their order of succession4 ,in the great mountain-chains1 especially1 it is exhibited on the grandest scale1 great groups of strata being folded over and over each other as we might fold carpets4 ,an inverted stratum is like a flattened ,s or ,z1 and may be pierced by a vertical shaft three times1 as has actually happened in some coal mines4 ,folds are open when the sides are not parallel1 and closed when they are parallel1 the former being represented by a half-open1 and the latter by a closed1 book4 ,closed folds are usually inverted1 and when the tops have been removed by erosion 7,fig4 #be71 the repetition of the strata may escape detection1 and the thickness of the section be1 in consequence1 greatly overestimated4 ,thus1 a geologist traversing the section in ,fig4 #be would see thirty-two strata1 all inclined to the left at the same angle1 those on the right apparently passing below those on the left1 and all forming part of one great fold4 ,the repetition of the strata in reverse order1 as indicated by the numbers1 and the structure below the surface1 show1 however1 that the section really consists of only four beds involved in a series of four closed folds1 the true thickness of the beds in this section being only one-eighth as great as the apparent thickness4 ,illustration3 ,fig4 #be4--,series of closed folds4 ,the most important features to be noted in observing and describing inclined or folded strata are the strike and dip4 ,the strike is the compass bearing or horizontal direction of the strata4 ,it is the direction of the outcrop of the strata where the ground is level4 ,it may also be defined as the direction of a level line on the surface of a stratum1 and is usually parallel with the axis of the fold4 ,illustration3 ,fig4 #bf4--;,dip and strike4 ,the dip is the inclination of the beds to the plane of the horizon1 and embraces two elements3 7a7 the direction of the dip1 which is always at right angles to the strike1 being the line of steepest descent on the surface of the stratum1 and 7b7 the amount of the dip1 which is the value of the angle between the line of steepest descent and the horizon4 ,in ,fig4 #bf1 s t is the direction of the strike1 and d p that of the dip4 ,the strike and direction of the dip are determined with the compass1 and the amount of the dip with the clinometer1 an instrument for measuring vertical angles4 ,the strike is much less variable than the dip1 being often essentially constant over extensive districts2 while the dip1 except in very large or closed folds1 is constantly changing in direction and amount4 ,when the dip and surface breadth of a series of strata have been measured1 it is a simple problem in trigonometry to determine the true thickness1 and the depth below the surface of any particular stratum at any given distance from its outcrop4 ,when the strata are vertical1 the surface breadth or traverse measure is equal to the thickness4 ,by the outcrop of a stratum or formation we ordinarily understand its actual exposure on the surface1 where it projects through the soil in ledges or quarries4 ,but the term is also more broadly defined to mean the exposure of the stratum as it would appear if the soil were entirely removed4 ,it is instructive to observe the relations of the outcrop to the form of the surface4 ,its breadth varies with its inclination to the surface1 appearing narrow and showing its true thickness where it is perpendicular to the surface1 and broadening out rapidly where the surface cuts it obliquely4 ,the outcrops of horizontal strata form level lines or bands along the sides of hills and valleys1 essentially contour lines in the topography2 and appear as irregular1 sinuous bands bordering the streams and valleys in the map-view of the country4 ,the outcrops of vertical strata1 dikes1 or veins1 on the other hand1 are represented by straight lines and bands on the map4 ,while the outcrops of inclined strata are deflected to the right or left in crossing ridges and valleys1 according to the direction and amount of their inclination4 ,a geological map shows the surface distribution of the rocks1 i4e41 gives in one view the forms and arrangement of the outcrops of all the rocks in the district mapped1 including the trend or strike of the folded strata4 ,the map may be lithological1 each kind of rock1 as granite1 sandstone1 limestone1 etc41 being represented by a different color2 or1 it may be historical1 each color representing one geological formation1 i4e41 the rocks formed during one period of geological time1 without reference to their lithological character4 ,but in the best maps these two methods are combined4 ,the geological section shows the arrangement of the rocks below the surface1 revealing the dip of the strata and supplementing the map1 both modes of representation1 the horizontal and vertical1 being required to give a complete idea of the geological structure of a country4 ,for a detailed and satisfactory explanation of the construction and use of geological maps and sections1 students are referred to ,prof4 ,geikie's 8,outlines of ,field ,geology40 ,,cleavage ,,structure4--,this important structure is now known to be1 like rock-folds1 a direct result of the great horizontal pressure in the earth's crust4 ,it is entirely distinct in its nature and origin from crystalline cleavage1 and may properly be called lithologic cleavage4 ,it is also essentially unlike stratification and joint-structure4 ,it agrees with stratification in dividing the rocks into thin parallel layers1 but the cleavage planes are normally vertical instead of horizontal4 ,and the cleavage planes differ from joints in running in only one direction1 dividing the rock into layers2 while joints1 as we shall see1 traverse the same mass of rock in various directions1 dividing it into blocks4 ,illustration3 ,fig4 #bg4--,slaty cleavage in contorted strata4 ,the principal characteristics of lithologic cleavage are3 7#a7 ,it is rare1 except in fine-grained1 soft rocks1 having its best development in the slates1 roofing slates and school slates affording typical examples4 ,hence it is commonly known as slaty cleavage4 7#b7 ,the cleavage planes are highly inclined or vertical1 very constant in dip and strike1 and quite independent of stratification4 7#c7 ,it is usually associated with folded strata1 and often with distorted nodules or fossils4 ,the more important of these characteristics are illustrated by ,fig4 #bg4 ,this represents a block of contorted strata in which the dark layers are slate with very perfect cleavage parallel to the left-hand shaded side of the block2 while the white layers are sandstone and quite destitute of cleavage4 ,many explanations of this interesting structure have been proposed1 but that first advanced by ,sharpe may be regarded as fully established4 ,he said that slaty cleavage is always due to powerful pressure at right angles to the planes of cleavage4 ,all the characteristics of cleavage noted above are in harmony with this theory4 ,cleavage is limited to fine-grained or soft rocks1 because these alone can be modified internally by pressure1 without rupture4 ,harder and more rigid rocks may be bent or broken1 but they appear insusceptible of minute wrinkling or other change of structure affecting every particle of the mass4 ,since the cleavage planes are normally vertical1 the pressure1 according to the theory1 must be horizontal4 ,that this horizontal pressure exists and is adequate in direction and amount1 is proved by the folds and contortions of the cleaved strata2 for1 as shown in ,fig4 #bg1 the cleavage planes coincide with the strike of the foldings1 and are thus perpendicular to the pressure horizontally as well as vertically4 ,the distortion of the fossils in cleaved slates is plainly due to pressure at right angles to the cleavage1 for they are compressed or shortened in that direction1 and extended or flattened out in the planes of cleavage4 ,again1 ,tyndall has shown that the magnetism of cleaved slate proves that it has been powerfully compressed perpendicularly to the cleavage4 ,and1 finally1 repeated experiments by ,sorby and others have proved that a very perfect cleavage may be developed in clay 7unconsolidated slate7 by compression1 the planes of cleavage being at right angles to the line of pressure4 ,when1 however1 ,sharpe's theory had been thus fully demonstrated1 the question as to how pressure produces cleavage still remained unanswered4 ,sorby held that clay contains foreign particles with unequal axes1 such as mica-scales1 etc41 and that these are turned by the pressure so as to lie in parallel planes perpendicular to its line of action1 thus producing easy splitting or cleavage in those planes4 ,and he proved by experiments that a mixture of clay and mica-scales does behave in this way4 ,but ,tyndall showed that the cleavage is more perfect just in proportion as the clay is free from foreign particles1 and in such a perfectly homogeneous substance as beeswax1 he developed a more perfect cleavage than is possible in clay4 ,his theory1 which is now universally accepted1 is1 that the clay itself is composed of grains which are flattened by pressure1 the granular structure with irregular fracture in all directions1 changing to a scaly structure with very easy and plane fracture or splitting in one definite direction4 ,observations on distorted fossils and nodules have shown that when slaty cleavage is developed1 the rock is1 on the average1 reduced in the direction of the pressure to two-fifths of its original extent1 and correspondingly extended in the vertical direction4 ,thus1 whether rocks yield to the horizontal pressure in the earth's crust1 by folding and corrugation1 or by the flattening of their constituent particles1 they are alike shortened horizontally and extended vertically2 and it is impossible to overestimate the importance of these facts in the formation of mountains4 ,,faults ,,or ,,displacements4--,we may readily conceive that the forces which were adequate to elevate1 corrugate1 and even crush vast masses of solid rock were also sufficient to crack and break them2 and since the fractures indicate that the strains have been applied unequally1 it will be seen that unequal movements of the parts must often result4 ,if this unequal movement takes place1 i4e41 if the rocks on opposite sides of a fracture of the earth's crust do not move together1 but slip over each other1 a fault is produced4 ,the two sides may move in opposite directions1 or in the same direction but unequally1 or one side may remain stationary while the other moves up or down4 ,it is simply essential that the movement should be unequal in direction1 or amount1 or both2 that there should be an actual slip1 so that strata that were once continuous no longer correspond in position1 but lie at different levels on opposite sides of the fracture4 ,the vertical difference in movement is known as the throw1 slip1 or displacement of the fault4 ,fault-fractures rarely approach the horizontal direction1 but are usually highly inclined or approximately vertical4 ,when the fault is inclined1 as in ,fig4 #bh1 the actual slipping in the plane of the fault exceeds the vertical throw1 for the movement is then partly horizontal1 the beds being pulled apart endwise4 ,the inclination of faults1 as of veins and dikes1 should be measured from the vertical and called the hade4 ,faults are sometimes hundreds of miles in length2 and the throw may vary from a fraction of an inch to thousands of feet4 ,illustration3 ,fig4 #bh4--,section of a normal fault4 ,illustration3 ,fig4 #bi4--,section of a reversed fault4 ,transverse sections1 such as are represented by ,fig4 #bh and many specimens and models1 do not give the complete plan or idea of a fault2 but this is seen more perfectly in ,fig4 #cj4 ,we learn from this that a typical fault is a fracture along which the strata have sagged or settled down unequally4 ,the most important point to be observed here is that the strata do not drop bodily1 but are merely bent1 the throw being greatest at the middle of the fault and gradually diminishing toward the ends4 ,in other words1 every simple fault must die out gradually2 for we cannot conceive of a fault as ending abruptly1 except where it turns upon itself so as to completely enclose a block of the strata1 which may drop down bodily2 but the fault is then really endless4 ,a fault may be represented on a map by a line2 if a simple fault1 by a single straight line4 ,but faults are often compound1 and are represented by branching lines2 that is1 the earth's crust has been broken irregularly1 and the parts adjoining the fracture have sagged or risen unequally4 ,illustration3 ,fig4 #cj4--;,ideal view of a complete fault4 ,the rock above an inclined fault1 vein1 or dike 7,fig4 #bh7 is called the hanging wall1 and that below the foot wall4 ,now inclined faults are divided into two classes1 according to the relative movements of the two walls4 ,usually1 the hanging wall slips down and the foot wall slips up1 as in ,fig4 #bh4 ,faults on this plan are so nearly the universal rule that they are called normal faults4 ,they indicate that the strata were in a state of tension1 for their broken ends are pulled apart horizontally1 so that a vertical line may cross the plane of a stratum without touching it4 ,a few important faults have been observed1 however1 in which the foot-wall9999no hyphen before has fallen and the hanging-wall9999 has risen 7,fig4 #bi74 ,these are known as reversed faults2 and they indicate that the strata were in a state of lateral compression1 the broken ends of the beds having been pushed horizontally past each other1 so that a vertical line or shaft may intersect the same bed twice1 as has been actually demonstrated in the case of some beds of coal4 ,illustration3 ,fig4 #ca4--;,explanation of normal faults4 ,the usual explanation of normal faults is given in ,fig4 #ca4 ,the inclined fractures of the earth's crust must often be converging1 bounding1 or enclosing large ,v-shaped blocks 7,a1 ,b74 ,if now1 through any cause1 as the folding of the strata1 they are brought into a state of tension1 so that the fractures are widened1 the ,v-shaped masses1 being unsupported1 settle down1 the fractures bounding them becoming normal faults1 as is seen by tracing the bed ,x through the dislocations4 ,the single fracture below the block ,a is inclined1 and the stretching has been accomplished by slipping along it and faulting the bed ,z as well as ,x1 the entire section to the right of this fracture being part of a much larger ,v-shaped block the right-hand boundary of which is not seen4 ,but the united fracture below the block ,b being vertical1 any horizontal movement must widen it into a fissure1 which is kept open by the great wedge above and may become the seat of a dike or mineral vein4 ,the beds below the ,v may1 in this case1 escape dislocation1 as is seen by tracing the bed ,z across the fissure4 ,these pairs of converging normal faults are called trough faults2 and this is the only way in which we can conceive of important faults as terminating at moderate depths below the surface1 and not affecting the entire thickness of the earth's crust4 ,important reversed faults are believed to occur chiefly along the axes of overturned anticlines 7,fig4 #bd7 where the strata have been broken by the unequal strains1 and those on the upper side shoved bodily over those on the lower or inverted side4 ,an extensive displacement of the strata is sometimes accomplished by short slips along each of a series of parallel fractures1 producing a step fault4 ,faults cutting inclined or folded strata are divided into two classes1 according as they are approximately parallel with the direction of the dip or of the strike4 ,the first are known as transverse or dip faults1 and the second as longitudinal or strike faults4 ,the chief interest of either class consists in their effect upon the outcrops of the faulted strata1 after erosion has removed the escarpment produced by the dislocation4 ,illustration3 ,fig4 #cb4--,plan of a dip fault4 ,dip faults cause a lateral shift or displacement of the outcrops1 as shown in ,fig4 #cb1 which represents a plan or map-view of the strata traversed by the fault b b1 the down throw being on the right and the up throw on the left4 ,the dip of the strata is indicated by the small arrows and the accompanying figures2 and it will be observed on tracing the outcrop of any stratum1 a a1 across the fault that it is shifted to the right4 ,if the throw of the fault were reversed1 the displacement of the outcrop would be reversed1 also4 ,strike faults are of two kinds1 according as they incline in the same direction as the strata1 or in the contrary direction4 ,the effect of the first kind is to conceal some of the beds1 as shown in ,fig4 #cc1 in which beds #e and #f do not outcrop1 but we pass on the surface abruptly from #d to #g4 ,the apparent thickness of the section is thus less than the real thickness4 ,when the fault inclines against the strata1 on the other hand 7,fig4 #cd71 the outcrops of certain strata are repeated on the surface2 and a number of parallel faults of this kind1 a step fault1 will1 like a series of closed folds 7,fig4 #be71 cause the apparent thickness of the section to greatly exceed the real thickness4 ,repetition of the strata by faulting is distinguished from repetition by folding by being in the same instead of the reverse order4 ,illustration3 ,fig4 #cc4--,strike fault1 concealing strata4 ,illustration3 ,fig4 #cd4--,strike fault1 repeating strata4 ,folds and faults are really closely related4 ,in the former the strata are disturbed and displaced by bending2 in the latter by breaking and slipping2 and the displacement which is accomplished by a fold may gradually change to a fracture and slip4 ,this relation is especially noticeable with monoclinal folds 7,fig4 #bc71 in which the tendency to shear or break the beds is often very marked4 ,important faults are rarely simple1 well-defined fractures2 but1 in consequence of the enormous friction1 the rocks are usually more or less broken and crushed1 sometimes for a breadth of many feet or yards4 ,the fragments of the various beds are then strung along the fault in the direction of the slipping1 and this circumstance has been made use of in tracing the continuation of faulted beds of coal4 ,in other cases the direction of the slip is plainly indicated by the bending of the broken ends of the strata 7,fig4 #ce71 and the beds are sometimes turned up at a high angle or even overturned in this way4 ,illustration3 ,fig4 #ce4--,section of beds distorted by a fault4 ,since faults are not plane1 but undulating and often highly irregular1 fractures1 the walls will not coincide after slipping2 and if the rocks are hard enough to resist the enormous pressure1 the cavities or fissures produced in this way may remain open4 ,now faults are continuous fractures of the earth's crust1 reaching down to an unknown but very great depth2 and hence they afford the best outlets for the heated subterranean waters2 so that it is common to find an important fault marked on the surface by a line of springs1 and these are often thermal4 ,the warm mineral waters on their way to the surface deposit part of the dissolved minerals in the irregular fissures along the fault1 which are thus changed to mineral veins4 ,this agrees with the fact that the walls of veins usually show faulting as well as crushed rock1 slickensides1 and other evidences of slipping4 ,if the earth's surface were not subject to erosion1 every fault would be marked on the surface by an escarpment equal in height to the throw of the fault2 and1 notwithstanding the powerful tendency of erosion to obliterate them1 these escarpments are sometimes observed1 although of diminished height4 ,thus1 according to ,gilbert1 the ,zandia ,mountains in ,new ,mexico are due to a fault of #aa1jjj feet1 leaving an escarpment still #gjjj feet high4 ,but1 as a rule1 there is no escarpment or marked inequality of the surface1 the fault1 like the fold1 not being distinctly indicated in the topography4 ,in all such cases we must conclude either that the faults were made a very long time ago1 or that they have been formed with extreme slowness1 so slowly that erosion has kept pace with the displacement1 the escarpments being worn away as fast as formed4 ,these and other considerations make it quite certain that extensive displacements are not produced suddenly1 but either grow by a slow1 creeping motion1 or by small slips many times repeated at long intervals of time4 ,,joints ,,and ,,joint-,,structure4--,this is the most universal of all rock-structures1 since all hard rocks and many imperfectly consolidated kinds1 like clay1 are jointed4 ,joints are cracks or planes of division which are usually approximately vertical and traverse the same mass of rock in several different directions4 ,they are distinguished from stratification planes by being rarely horizontal1 and from both stratification and cleavage planes by being actual cracks or fractures1 and by dividing the rock into blocks instead of sheets or layers4 ,the art of quarrying consists in removing these natural blocks2 and most of the broad flat surfaces of rock exposed in quarries1 are the joint-planes 7,fig4 #cf74 ,some of the most familiar features of rock-scenery are also due to this structure1 cliffs1 ravines1 etc41 being largely determined in form and direction by the principal systems of joints2 and we have already seen that the same is true of veins and dikes4 ,joints are divided by their characteristics and modes of origin into three classes as follows3-- ,illustration3 ,fig4 #cf4--,quarry showing two systems of parallel joints4 #a4 ,the parallel and intersecting joints4--,this is by far the most important class1 and has its best development in stratified rocks1 such as sandstone1 slate1 limestone1 etc4 ,these joints are straight and continuous cracks which may often be traced for considerable distances on the surface4 ,they usually run in several definite directions1 being arranged in sets or systems by their parallelism4 ,thus in ,fig4 #cf one set of joints is represented by the broad1 flat surfaces in light1 and a second set crossing the first nearly at right angles1 by the narrower faces in shadow4 ,by the intersections of the different sets of joints the rock is divided into angular blocks4 ,although many explanations of this class of joints have been proposed1 it has long been the general opinion of geologists that they are due to the contraction of the rocks1 i4e41 that they are shrinkage cracks4 ,we shall soon see1 however1 that they lack the most important characters of cracks known to be due to shrinkage2 and the present writer has advanced the view that movements of the earth's crust1 and especially the swift1 vibratory movements known as earthquakes1 are a far more adequate and probable cause4 ,it is well known that earthquakes break the rocks2 and1 if space permitted1 it could be shown that the earthquake-fractures must possess all the essential features of parallel and intersecting joints4 ,illustration3 ,fig4 #cg4--;,columnar dike4 #b4 ,contraction joints or shrinkage cracks4--,that many cracks in rocks are due to shrinkage1 there can be no doubt4 ,the shrinkage may result from the drying of sedimentary rocks2 but more generally from the cooling of eruptive rocks4 ,every one has noticed in warm weather1 the cracks in layers of mud or clay on the shore1 or where pools of water have dried up2 and we have already seen that these sun-cracks are often preserved in the hard rocks4 ,they have certain characteristic features by which they may be distinguished from the joints of the first class4 ,they divide the clay into irregular1 angular blocks1 which often show a tendency to be hexagonal instead of quadrangular4 ,the cracks are continually uniting and dividing1 but are not parallel1 and rarely cross each other4 ,sun-cracks never affect more than a few feet in thickness of clay1 and are an insignificant structural feature of sedimentary rocks4 ,in eruptive rocks1 on the other hand1 the contraction joints have a very extensive1 and1 in some cases1 a very perfect development1 culminating in the prismatic or columnar jointing of the basaltic rocks4 ,this remarkable structure has long excited the interest of geologists1 and1 although the basalt columns were once regarded as crystals1 and later as a species of concretionary structure1 it is now generally recognized as the normal result of slow cooling in a homogeneous1 brittle mass4 ,the columns are normally hexagonal1 and perpendicular to the cooling surface1 being vertical in horizontal sheets and lava flows1 as in the classic examples of the ,giant's ,causeway and ,fingal's ,cave1 and horizontal in vertical dikes 7,fig4 #cg74 ,they begin to grow on the cooling surface of the mass and gradually extend toward the centre1 so that dikes frequently show two independent sets of columns4 #c4 ,the concentric joints of granitic rocks4--,in quarries of granite and other massive crystalline rocks1 it is often very noticeable that the rock is divided into more or less regular layers by cracks which are approximately parallel with the surface of the ground1 some of the granite hills having thus a structure resembling that of an onion4 ,the layers are thin near the surface1 become thicker and less distinct downwards1 and cannot usually be traced below a depth of fifty or sixty feet4 ,these concentric cracks are of great assistance in quarrying1 and are now regarded as due to the expansion of the superficial portions of the granite caused by the heat of the sun4 ,in reference to this view of their origin these may be properly called expansion joints4 ,,structure ,,of ,,mountain-,,chains4--,mountains are primarily of two kinds1--volcanic and non-volcanic4 ,the structure of the former belongs properly with the original structures of the volcanic rocks2 but the latter--the true mountains--owe their internal structure and altitude or relief almost wholly to the crumpling and mashing together of great zones of the earth's crust1 being1 as already pointed out1 the culminating points of the plication1 cleavage1 and faulting of the strata4 8,a mountain-chain consists of a great plateau or bulge of the earth's surface1 often hundreds of miles wide and thousands of miles long4 ,this is usually more or less distinctly divided by great longitudinal valleys into parallel ranges and ridges2 and these1 again1 are serrated along their crests1 or divided into peaks by transverse valleys4 ,in many cases this ideal chain is far from realized1 but we have instead1 a great bulging of the earth's crust composed on the surface of an inextricable tangle of ridges and valleys of erosion1 running in all directions4 ,in all cases1 however1 the erosion has been immense2 for the mountain-chains are the great theatres of erosion as well as of igneous action4 ,as a general fact1 all that we see1 when we stand on a mountain-chain--every peak and valley1 every ridge and caon1 all that constitutes scenery--is wholly due to erosion40--,,le ,,conte4 ,the structure of mountains thus fells under two heads3 7#a7 ,the internal structure and altitude1 which are due to the action of the subterranean agencies4 7#b7 ,the external forms1 the actual relief1 which are the product chiefly of the superficial agencies or erosion4 ,the study of mountains has shown that3 7#a7 ,they are composed of very thick sedimentary formations4 ,thus the sedimentary rocks have a thickness of #dj1jjj feet in the ,alleghanies2 of #ej1jjj feet in the ,alps2 and of two to ten miles in all important mountain-chains4 ,such thick deposits of sediments1 as we have already seen1 must be formed on a subsiding sea-floor1 and in many mountain-chains1 as in the ,alleghanies1 the great bulk of these sediments are still below the level of the sea4 ,again1 thick sedimentary deposits can only be formed in the shallow1 marginal portions of the sea2 and when such a belt of thick shore deposits yields to the powerful horizontal thrust1 and is crumpled and mashed up1 it is greatly shortened in the direction of the pressure and thickened vertically1 so that its upper surface is lifted high above the level of the sea1 and a mountain-chain is formed and added to the edge of the continent4 ,we thus find an explanation of the important fact that on the several continents1 but notably on the two ,americas1 the principal mountain-ranges are near to and parallel with the coast lines4 #b4 ,the mountain-forming sediments are usually strongly folded and faulted1 and exhibit slaty cleavage wherever they are susceptible of that structure2 and the older rocks1 especially1 in mountains are often highly metamorphosed1 and are traversed by numerous veins and dikes1 the infallible signs of intense igneous activity4 8,in other words1 mountain regions have been the great theatres--7#a7 of sedimentation before the mountains were formed2 7#b7 of plication and upheaval in the formation of the range2 and 7#c7 of erosion which determined the present outline4 ,add to these the metamorphism1 the faults1 veins1 dikes1 and volcanic outbursts1 and it is seen that all geological agencies concentrate there40--,,le ,,conte4 ,since mountain-ranges are great up-swellings or bulgings of the strata1 their structure is always essentially anticlinal2 and they sometimes consist of a single more or less denuded anticline 7,fig4 #ch71 the oldest and lowest strata exposed forming the summit of the range4 ,more commonly1 however1 the single great arch or uplift is modified by a series of longitudinal folds1 as shown in the section of the ,jura ,mountains 7,fig4 #ba74 ,still more commonly the folds are closely pressed together1 overturned1 broken1 and almost inextricably complicated by smaller folds1 contortions1 and slips4 ,illustration3 ,fig4 #ch4--;,anticlinal mountain4 ,the strata on the flanks of the mountains are usually less disturbed than those near the axis of the range1 and are sometimes seen to rest unconformably against the latter4 ,in this way it is proved that some ranges are formed by successive upheavals4 ,but we have still more conclusive evidence that mountains are formed with extreme slowness in the fact that rivers sometimes cut directly through important ranges4 ,this proves1 first1 that the river is older than the mountains2 second1 that the deepening of its channel has always kept pace with the elevation of the range4 ,,concretions ,,and ,,concretionary ,,structure4--,folds1 cleavage1 faults1 and joints--all the subsequent structures considered up to this point--are the product of mechanical forces4 ,chemical agencies1 although very efficient in altering the composition and texture of rocks1 are almost powerless as regards the development of rock-structures2 and the only important structure having a chemical origin is that named above4 ,concretions are formed by the segregation of one or more of the constituents of a rock4 ,but there are three distinct kinds of segregation4 ,if the water percolating through or pervading a rock1 dissolves a certain mineral and afterwards deposits it in cavities or fissures1 amygdules1 geodes1 or veins are the result4 ,if the mineral is deposited about particular points in the mass of the rock1 it may form crystals1 the rock becoming porphyritic2 or it may not crystallize1 but build up instead the rounded forms called concretions1 the texture or structure of the rock becoming concretionary4 ,a great variety of minerals occur in the form of concretions1 but this mode of occurrence is especially characteristic of certain constituents of rocks1 such as calcite1 siderite1 limonite1 hematite1 and quartz4 ,concretions may be classified according to the nature of the segregating minerals2 and in each class we may distinguish the pure from the impure concretions4 ,a pure concretion is one entirely composed of the segregating mineral4 ,most nodules of flint and chert1 quartz1 geodes1 concretions of pyrite1 and many hollow iron-balls are good illustrations of this class4 ,in all these cases the segregating mineral has been able in some way to remove the other constituents of the rock1 and make room for itself4 ,but in other cases it has lacked this power1 and has been deposited between and around the grains of sand1 clay1 etc42 and the concretions are consequently impure1 being composed partly of the segregating mineral1 and partly of the other constituents of the rock4 ,the calcareous concretions known as clay-stones are a good example of this class1 being simply discs of clay1 all the minute interstices of which have been filled with segregated calcite4 ,the solid iron-balls are masses of sand filled in a similar manner with iron oxides4 ,concretions are of all sizes1 from those of microscopic smallness in some olitic limestones up to those twenty-five feet or more in diameter in some sandstones4 ,the point of deposition1 when a concretion begins to grow1 is often determined by some concrete particle1 as a grain or crystal of the same or a different mineral1 a fragment of a shell1 or a bit of vegetation1 which thus becomes the nucleus of the concretion4 ,the ideal or typical concretion is spherical2 but the form is influenced largely by the structure of the rock4 ,in porous rocks1 like sandstone1 they are frequently very perfect spheres2 but in impervious rocks1 like clay1 they are flat or disc-shaped1 because the water passes much more freely in the direction of the bedding than across it2 while the concretions in limestones1 the nodules of flint and chert1 are often remarkable for the irregularity of their forms4 ,in all sedimentary rocks the concretions are arranged more or less distinctly in layers parallel with the stratification1 which usually passes undisturbed through the impure concretions4 ,many silicious and ferruginous concretions are hollow1 apparently in consequence of the contraction of the substance after its segregation2 and the shrinkage due to drying is still further indicated by the cracks in the septaria stones4 ,the hollow1 silicious concretions are usually lined with crystals 7geodes71 while the hollow iron-balls frequently enclose a smaller concretion4 ,rocks often have a concretionary structure when there are no distinct or separable concretions4 ,and the appearance of a concretionary structure 7pseudo-concretions7 is often the result of the concentric decomposition of the rocks by weathering1 as explained on page #ac4 ,,subsequent ,,structures ,,produced ,,by ,,the ,,superficial ,,or ,,aqueous ,,agencies4--,the superficial agencies1 as we have seen in the section on dynamical geology1 are1 in general terms1 water1 air1 and organic matter4 ,geologically considered1 the results which they accomplish1 may be summed up under the two heads of deposition and erosion--the formation of new rocks in the sea1 and the destruction of old rocks on the land4 ,in the rle of rock-makers they produce the very important original structures of the stratified rocks2 while as agents of erosion they develop the most salient of the subsequent structures of the earth's crust--the infinitely varied relief of its surface4 ,as a general rule1 to which recent volcanoes are one important exception1 the original and subterranean structures of rocks are only indirectly1 and often very slightly1 represented in the topography2 for this1 as we have seen1 is almost wholly the product of erosion4 ,therefore1 what we have chiefly to consider in this section is to what extent and how erosion is influenced by the pre-existing structures of rocks4 ,horizontal or very slightly undulating strata1 especially if the upper beds are harder than those below1 give rise by erosion to flat-topped ridges or table-mountains 7,fig4 #ci74 ,but if the strata be softer and of more uniform texture1 erosion yields rounded hills1 often very steep1 and sometimes passing into pinnacles1 as in the ,bad ,lands of the west4 ,broad1 open folds1 as we have seen1 give1 normally1 synclinal hills and anticlinal valleys 7,fig4 #bb71 when the erosion is well advanced4 ,but in more strongly1 closely folded rocks the ridges and valleys are determined chiefly by the outcrops of harder and softer strata1 as shown in ,fig4 #dj1 the symmetry of the reliefs depending upon the dip of the strata4 ,this principle of unequal hardness or durability also determines most of the topographic features in regions of metamorphic and crystalline rocks1 in which the stratification is obscure or wanting4 ,illustration3 ,fig4 #ci4--;,horizontal strata and table-mountains4 ,illustration3 ,fig4 #dj4--,ridges due to the outcrops of hard strata4 ,the boldness of the topography1 and the relation of depth to width in valleys1 depends largely upon the altitude above the sea2 but partly1 also1 upon the distribution of the rainfall1 the drainage channels or valleys being narrowest and most sharply defined in arid regions traversed by rivers deriving their waters from distant mountains4 ,that these are the conditions most favorable for the formation of caons is proved by the fact that they are fully realized in the great plateau country traversed by the ,colorado and its tributaries1 a district which leads the world in the magnitude and grandeur of its caons4 ,but deep gorges and caons will be formed wherever a considerable altitude1 by increasing the erosive power of the streams1 enables them to deepen their channels much more rapidly than the general face of the country is lowered by rain and frost4 ,this is the secret of such caons as the ,yosemite ,valley1 and the gorge of the ,columbia ,river1 and probably of the fiords which fret the north-west coasts of this continent and ,europe4 ,for a full description and illustration of the topographic types developed by the action of water and ice upon the surface of the land1 and of the various characteristic forms of marine erosion1 teachers are referred to the larger works named in the introduction1 especially ,le ,conte's ,elements of ,geology1 and to the better works on physical geography4 ,we will1 in closing this section1 merely glance at some of the minor erosion-forms1 which are not properly topographic1 but may be often illustrated by class-room and museum specimens4 ,mere weathering1 the action of rain and frost1 develops very characteristic surfaces upon different classes of rocks1 delicately and accurately expressing in relief those slight differences in texture1 hardness1 and solubility1 which must exist even in the most homogeneous rocks4 ,every one recognizes on sight the hard1 smooth surfaces of water-worn rocks4 ,they are exemplified in beach and river pebbles1 in sea-worn cliffs1 and where rivers flow over the solid ledges4 ,the pot-hole 7page #ag7 is a well-marked and specially interesting rock-form1 due to current or river erosion4 ,ice has also left highly characteristic traces upon the rocks in all latitudes covered by the great ice-sheet4 ,these consist chiefly of polished1 grooved1 and scratched or striated surfaces1 the grooves and scratches showing the direction in which the ice moved4 ,the organic agencies1 as already noted1 accomplish very little in the way of erosion1 especially in the hard rocks1 but the rock-borings made by certain mollusks and echinoderms may be mentioned as one unimportant but characteristic form due to organic erosion4 ---------------------------------------- -------------------------------- ,,appendix ,the following collections are especially prepared and arranged for use with this text3 ,weathering #a ,diabase 99#b 8 weathered 99#c 8 disintegrated #d ,felsite3 ,angular fragment #e 8 ,water rounded pebble ,formation of ,coals 99#f ,peat #g ,lignite #h ,bituminous 99#i ,cannel coal #aj ,anthracite #aa ,native coke ,rock-forming ,minerals 99#ab ,graphite #ac ,halite 99#ad ,limonite 99#ae ,hematite 99#af ,magnetite #ag 8 ,lodestone 99#ah ,quartz3 ,glassy #ai 8 ,flint #bj 8 ,chert #ba ,opalized wood 99#bb ,gypsum 99#bc ,calcite #bd ,dolomite #be ,siderite 99#bf ,hornblende #bg ,pyroxene 99#bh ,muscovite #bi ,biotite 99#cj ,orthoclase #ca ,albite 99#cb ,labradorite 99#cc ,kaolinite #cd ,talc 99#ce ,serpentine #cf ,chlorite #cg ,glauconite 7,green ,sand7 #ch ,chrysolite #ci ,garnet #dj ,pyrite ,sedimentary and ,metamorphic ,rocks 99#da ,conglomerate3 ,breccia 99#db 8 ,pudding-stone 99#dc ,sand3 ,quartz #dd 8 ,magnetite 99#de ,sandstone3 ,ferruginous #df 8 ,calcareous #dg 8 ,arkose 99#dh ,quartzite #di ,clay3 ,boulder #ej 8 ,fire 99#ea ,shale 99#eb 8 ,carbonaceous #ec ,slate3 ,roofing #ed 8 ,flagstone #ee ,porcelainite #ef ,tripolite #eg ,siliceous ,tufa #eh ,novaculite #ei ,asphaltum #fj ,oil ,sand 99#fa ,limestone3 ,fossiliferous 99#fb 8 ,coquina 99#fc 8 ,chalk #fd 8 ,crystalline #fe 8 ,compact #ff 8 ,hydraulic #fg ,calcareous ,tufa #fh ,dolomite #fi ,rock ,salt #gj ,phosphate ,nodule 99#ga ,gneiss3 ,granitoid 99#gb 8 ,micaceous #gc 8 ,hornblendic #gd ,norite3 ,hypersthenite 99#ge ,schist3 ,mica #gf 8 ,hornblende #gg 8 ,talc #gh 8 ,chlorite #gi ,amphibolite #hj ,soapstone #ha ,verd ,antique 7,serpentine7 ,igneous ,rocks 99#hb ,granite3 ,binary #hc 8 ,muscovite 99#hd 8 ,biotite #he 8 ,hornblendic #hf 8 ,red 99#hg ,syenite #hh 8 ,elolite 99#hi ,diorite 99#ij ,diabase3 ,trap 99#ia ,rhyolite #ib ,trachyte 99#ic ,obsidian #id ,pumice #ie ,petrosilex 99#if ,andesite 99#ig ,basalt #ih 8 vesicular ,lava 99#ii ,melaphyr3 ,amygdaloidal #ajj ,volcanic ,tuff ,collection ,no4 ,f#a4 ,entire list of #ajj museum size specimens 7#c #d71 numbered1 labelled and mounted on blocks or in improved trays1 for museum display and laboratory work #dj4jj 7,the same1 labelled but unmounted1 #cj4jj7 ,collection ,no4 ,f#b4 ,same as above1 but small museum size1 mounted in improved trays 7#b #c7 #be4jj ,collection ,no4 ,f#c4 ,same as ,f#b1 but hand size specimens 7#b #b7 #ab4ej ,collection ,no4 ,f#d4 #hj specimens1 omitting those marked 771 in individual trays 7#b #a7 and two cloth-board cases1 numbered to correspond with accompanying printed list 7no labels7 #e4jj ,collection ,no4 ,f#e4 #dj specimens marked 79971 mounted as collection ,f#d #b4ej ,collection ,no4 ,f#f4 #ajj pupils' fragments 7#a #a71 numbered1 in paper bags4 7,single collection #a4be47 ,in lots of #e or more1 each #a4jj ,collection ,no4 ,f#g4 #hj pupils' fragments 7like ,f#f74 7,single #a4jj47 ,in lots of #e or more1 each 4#ge ,collection ,no4 ,f#h4 #dj pupils' fragments 7like ,f#f74 7,single #ej;c47 ,in lots of #e or more1 each 4#dj ,collection ,no4 ,f#i4 #be museum size specimens1 illustrating structure1 faults1 stratification1 etc4 ,mounted and labelled #aj4jj ,for further information or in ordering1 address ,,ward',s ,,natural ,,science ,,establishment #hd-ajb ,college ,ave41 ,rochester1 ,n4 ,y4 ---------------------------------------- -------------------------------- ,transcriber's note3 ,specimen numbers have been regularised as medium weight4 ,page #g1 #8,le,conte' changed to #8,le ,conte1' 8,by ,prof4 ,joseph ,le ,conte40 ,page #ab1 #8contined' changed to #8continued1' 8continued exposure to the0 ,page #bh1 comma changed to full stop1 8associated with beds of coal40 ,page #cd1 #8or' changed to upright1 8superficial or aqueous agencies0 ,page #if1 all instances of #8per cent' changed to #8per cent4'3 8#fj per cent401 8#cj per cent401 8#he per cent40 ,page #ig1 full stop inserted after #8crust1' 8of the earth's crust4 ,the name0 ,page #aje1 specimen number absent in original4 ,page #ajf1 #8green sand' changed to #8greensand1' 8greensand1 and serpentine0 ,page #ajg1 #8magnesion' changed to #8magnesian1' 8the entire rock is magnesian10 ,page #ajh1 #8#gj' changed to #8#fi1' 8classification on page #fi0 ,page #aad1 full stop inserted after #8rocks1' 8the stratified rocks4 ,like0 ,page #abf1 full stop inserted after #8veins1' 8in veins4 ,those fissures0 ,page #adf1 instance of thousands formatted without comma delimiter3 8#djjj to #ejjj feet0 ,page #ahh1 instance of thousands formatted without comma delimiter3 8#gjjj feet high0