,transcrib]'s ,notes3 ,small capitals h be5 conv]t$ to ,,solid capitals4 ,illu/rations h be5 mov$ s !y d n break up p>agraphs4 ,typographical & punctuation errors h be5 sil5tly correct$4 ,,h[ ,,old ,,are ,,fossils8 ,,by ,,%>at ,k4 ,,roy ,,assi/ant ,,curator ,,( ,,inv]tebrate ,,paleontology ,illu/ration ,,geology ,,leaflet #i ,,field ,,museum ,,( ,,natural ,,hi/ory ,,*icago #aibg ,,li/ ,,( ,,geological ,,leaflets ,,issu$ ,,to ,,date ,no4 #a4 ,model ( an ,arizona ,gold ,m9e 4#aj ,no4 #b4 ,models ( ,bla/ ,furnaces = ,smelt+ ,iron 4#aj ,no4 #c4 ,amb]--,its ,physical ,prop]ties & ,geological ,occurr5ce 4#aj ,no4 #d4 ,meteorites 4#aj ,no4 #e4 ,soils 4#aj ,no4 #f4 ,! ,moon 4#aj ,no4 #g4 ,e>ly ,geological ,hi/ory ( ,*icago 4#be ,no4 #h4 ,agate 4#ej ,no4 #i4 ,h[ old are ,fossils8 4#be ,d4 ,c4 ,,davies1 ,,director ,,field ,,museum ,,( ,,natural ,,hi/ory ,,*icago1 ,u4,s4,a4 ,illu/ration3 ,,leaflet #i4 ,,plate ,i4 ,a ,,view ,,( ,,! ,,gr& ,,canyon ,,( ,,! ,,colorado ,,riv]1 ,,arizona4 ,,zoroa/] ,,9 ,,backgr\nd4 ,photograph by ,o4 ,c4 ,f>r+ton4 ,,field ,,museum ,,( ,,natural ,,hi/ory ,,dep>tm5t ,,( ,,geology ,,*icago1 #aibg ,,leaflet ,,numb] #i ,h[ old are ,fossils8 8,h[ old is t fossil & h[ d y "k x80 is a "q frequ5tly ask$ by visitors go+ ?r\gh ! hall ( fossils 9 ! ,museum4 ,a precise answ] to su* a "q is impossible & an adequate "o dem&s a long] "t ?an c usually be af=d$4 ,! consequ5ces ( inadequate explanation h (t5 prov$ to be unsatisfactory4 ,! visitor becomes skeptical & in/ead ( tak+ int]e/ 9 ! subject1 he seems to be confirm$ 9 8 d\bts4 ,9 ?is leaflet is giv5 a cond5s$1 g5]al /atem5t ( me?ods ( det]m9+ ! age ( anci5t life4 ,! in=mation is drawn f ! works ( v>i\s au?ors1 especially ,b>rell's 8,rhy?ms & ,measurem5ts ( ,geologic ,"t40 ,x is int5d$ = ^? :o are int]e/$ 9 ! age ( pa/ life & yet d n int5d an exhau/ive /udy ( ! subject1 nor h free & easy access to its lit]ature4 ,! bir? ( life 0 ! mo/ mom5t\s occasion 9 ! hi/ory ( ! e>?4 ,:5 "o consid]s ! myriads ( evid5ces une>!d by paleontologi/s & paleobotani/s1 !y seem to leave no room to d\bt ! great conception t ! life ( ! l& has em]g$ f ! sea4 ,x is1 !re=e1 only a natural impulse to look = ! rema9s ( ?is life 9 ! rocks laid d[n by ! anci5t seas & to wond] at ! va/ness ( "t beh9d !m4 ,s9ce traces ( ! l[e/ =ms ( life h be5 f\nd 9 practically ! olde/ kn[n s$im5t>y /rata1 ! problem ( det]m9+ ! age ( life necess>ily involves ! det]m9ation ( ! age ( ^? /rata4 ,b unravell+ ! dead pa/ is n an easy task4 ,"o try+ to unlock ! 8secrets ( ! cemet]y ( ,nature's dead10 walks on a %ad[y road4 ,8 difficulties are _m4 ,x is l cross+ a deep moat1 climb+ a /eep wall4 ,v>i\s me?ods h be5 appli$ to e/imate ! age ( diff]5t p]iods ( ! e>?'s hi/ory & mu* progress has be5 made t[>d a successful issue4 ,broadly1 ! proc$ure ( diff]5t me?ods is ! same4 ,!y d n diff] 9 pr9ciple4 8,! rates ( c]ta9 *anges at ! pres5t day are det]m9$ z accurately z possible1 & 9 imag9ation1 ! respective processes are trac$ backw>d 9 "t until limit+ conditions are arriv$ at40 ,until ! epo*-mak+ discov]y ( radium1 ! two mo/ out/&+ me?ods us$ 9 calculat+ geologic "t 7 7#a7 ! rate ( l& erosion & deposition & 7#b7 ! rate ( d]ivation ( salt 7sodium *loride7 f ! l& & its accumulation 9 ! oceans4 ,!oretically1 x is simple to use ! rate at :i* s$im5ts are be+ deposit$ or solutions ga!r$ into ! ocean1 z 8geologic clocks0 = e/imat+ ! l5g? ( pa/ "t4 ,b 9 practice ea* me?od enc\nt]s its own difficulties & ! results d$uc$ give u at be/ only a r\gh idea ( ! imm5sity ( "t involv$4 ,i %all n deal ) ^! individual e/imates1 b give ! mean ( sev]al1 :i* is #ajj1jjj1jjj ye>s1 speak+ r\ndly4 ,z /at$ be=e1 x is only a r\gh e/imate4 ,nev]!less1 x confirms ! fact t ! e>? is v old--inde$ mu* old] ?an is commonly believ$4 ,9 #afej1 ,bi%op ,us%]1 9 8 int]pretation ( ! 8,9 ! beg9n+0 ( ,g5esis1 e/imat$ t ! e>? 0 creat$ #djjd ye>s be=e ! bir? ( ,*ri/4 ,accord+ to ?is view ! e>? is #eica ye>s old today4 ,_m cosmogoni/s & ev5 "s geologi/s ( ! #ai? c5tury held ?is ,biblical int]pretation to be ! age ( ! e>?4 ,o!r anci5t religions held t ! e>? 0 creat$ mu* e>li] !n #djjd ,b4,c4 ,hutton1 "o ( ! f\nd]s ( ,geology1 9 8 /udies1 f\nd 8no ve/ige ( a beg9n+--no prospect ( an end40 ,"o _c help sympa?iz+ ) ,hutton4 ,:oev] has made a trip to ! ,gr& ,canyon ( ! ,colorado ,riv]1 ,arizona1 mu/ rememb] ! awe-inspir+ dep? ( ! ,paleozoic /rata & ?\s&s ( feet ( ,prot]ozoic s$im5ts b5ea? !m 7,pl4 ,i74 ,if he has travel$ f>!r nor? to ! ,cab9et ,range1 ,montana1 he mu/ h c>ri$ ) him an undy+ impression ( ! #ce1jjj ft4 ( ! rocky monum5t1 !re built up by ! ,prot]ozoic seas4 ,illu/ration3 ,,leaflet #i4 ,,plate ,,ii4 ,,comp>ison ,,( ,,an ,,anci5t ,,& ,a ,,mod]n ,,m>9e ,,animal ,,( ,,alli$ ,,gr\ps4 #a1 ,a ,,cambrian ,,trilobite 7,,aft] ,,walcott74 #b1 ,a ,,mod]n ,,horse%oe ,,crab4 ,! pres5t rate ( d5udation 9 ! ,hudson ,bay region is "o foot 9 ab\t #dg1jjj ye>s4 ,h[ long !n1 has x tak5 ! seas to lay d[n ^! miles ( s$im5ts1 :i* are b a small fraction ( ! :ole geologic column8 ,hav+ consid]$ all ?is1 c we e/imate1 ev5 approximately1 ! va/ l5g? ( "t t has elaps$ betwe5 ^! p]iods ( s$im5tation1 ! s-call$ 8gaps0 or 8breaks0--! torn1 illegibly writt5 pages ( ! hi/ory ( ! e>?8 ,inde$1 !re are mom5ts :5 all may feel t x is mu* beyond _! compreh5sion4 ,b1 man1 by nature1 is at once humble & exalt$4 ,he is will+ to admit 8 defeat1 yet 8 ?ir/ to conqu] new k1 to "k ! tru?1 is nev] satiat$4 ,! reason = ! failure to arrive at an absolute result is n v f> to seek4 ,9 comput+ geologic "t1 "o has to calculate t :i* has elaps$1 by "s process 9 nature t takes place 9 "o direction only & t does n *ange its rate :5 conditions alt]4 ,:atev] ! me?od appli$1 be x ! deposition ( s$im5ts1 or ! ga!r+ ( solutions1 or ! los+ ( heat by ! sun & ! e>?1 its rate ( action %\ld be uni=m & un9t]rupt$4 ,x %\ld be indep5d5t ( ! *ang+ conditions ( ! e>?4 ,uni=mity ( ! rate ( action is ! crit]ion = precise calculation4 ,b we "k t ! pa/ 0 q diff]5t f ! pres5t4 ,diff]5t conditions h exi/$ at diff]5t times dur+ ! e>?'s hi/ory4 ,! configuration ( ! e>?1 its climate1 humidity1 temp]ature & _m o!r factors h v>i$ f "t to "t & ) !m ! rate ( erosion1 deposition & solution has ei!r accel]at$1 dim9i%$ or ceas$4 ,! doctr9e ( uni=mit>ianism _c be assum$ 9 a *ang+ _w1 ev5 ?\gh our k ( ! e>? ( ! pa/ c only be ga9$ f a full] /udy ( ! e>? ( ! pres5t4 ,:5 we consid] ! rate ( s$im5tation z a me?od = e/imat+ geologic "t1 we take ! total obs]v$ ?ickness ( ! geologic column1 7e/imat$ to be #gj miles7 & divide x by ! rate at :i* ! s$im5ts are n[ be+ laid d[n4 ,b d we "k ?is rate8 ,by tak+ ! av]age rate ( s$im5tation ( n9e l>ge riv]s#a n[ 9 exi/5ce & assum+ x to be ! rate at :i* s$im5ts 7 deposit$ 9 ! pa/1 an approximate conclusion c be arriv$ at4 ,b x w be notic$ f ! figures ( ! rate ( s$im5tation ( diff]5t riv]s t !y are widely v>iable1 ! highe/ be+ _m times great] ?an ! l[e/4 ,c we !n use an av]age ( n9e figures s out ( proportion & yet expect a reliable quantitative value8 ,fur!rmore1 ! rate ( deposition along ! coa/1 ne> ! m\? ( a l>ge & active riv]1 is mu* high] ?an on a coa/l9e :]e no riv]s empty _! s$im5ts into ! sea4 ,! rate is also l>gely controll$ by ! "* ( ! s$im5ts deposit$4 ,s&/one & %ale are m rapidly deposit$ ?an lime/one4 ,moreov]1 j z !re is no k ( ! duration ( "t ( erosion betwe5 p]iods ( s$im5tation1 !re is also no record ( ! am\nt ( detritus t has fall5 (f ! edge ( ! cont95tal %elf dur+ widespread em]g5ce ( ! cont95ts4 #a ---------------------------------------- ------------------------ ,riv] ,dra9age ,total tons ,ratio ( ,height 9 feet ( areas 9 annually s$im5t column ( s$im5t squ>e to wat] ) a base ( miles 9 weight #a squ>e mile ---------------------------------------- ------------------------ ,potomac #aa1jdc #e1eeg1bej #a3 #c1ege #d4j ,mississippi #a1bdd1jjj #djf1bej1jjj #a3 #a1ejj #bda4d ,rio ,gr&e #cj1jjj #c1hcj1jjj #a3 #bia #b4h ,uruguay #aej1jjj #ad1ghb1ejj #a3aj1jjj #aj4f ,rhone #cd1hjj #cf1jjj1jjj #a3 #a1gge #ca4a ,po #bg1ajj #fg1jjj1jjj #a3 #ijj #ei4j ,danube #cbj1cjj #ajh1jjj1jjj #a3 #b1hjj #ic4b ,nile #a1ajj1jjj #ed1jjj1jjj #a3 #b1jej #ch4h ,irrawaddy #abe1jjj #bia1dcj1jjj #a3 #a1faj #bji4j ---------------------------------------- ------------------------ ,babb4 ,sci5ce1 ,vol4 ,,xxi1 p4 #cdc1 #ahic4 ,we h also no record ( ! va/ quantity ( %all[ wat] s$im5ts t 7 /irr$ up by ! p5etration ( /orm waves1 & c>ri$ to abyssal dep?s by ! curr5ts & tides4 ,simil> unc]ta9ties beset u :5 we consid] ! rate ( *emical d5udation1 t is1 ! rate at :i* salts h be5 dissolv$ f ! l&s & accumulat$ 9 ! oceans1 z a measure ( geological "t4 ,"h1 aga91 we take ! total am\nt ( salts t is 9 ! oceans today & divide x by ! pres5t rate ( annual supply4 ,we "k ) reasonable accuracy ! quantity ( salts 9 ! oceans & if x 7 possible to assume ! pres5t rate ( supply to be a true mean = all geological "t1 a satisfactory age ( ! oceans might be obta9$4 ,b x _c be assum$ z su*4 ,an assumption ( ?is nature w only lead u f ! doma9 ( exactness to t ( unc]ta9ty4 ,aside f v>i\s o!r factors1 nei!r ! area ( ! cont95ts1 nor _! relief 0 9 ! pa/ ! same z today4 ,consequ5tly1 ! /ream gradi5t & its p[] ( dissolv+ salts f ! l& surfaces h n be5 ! same4 ,x is also n kn[n h[ mu* salt ! ocean d]iv$ f ! %ore l9e & f b$s b5ea? ! ocean1 nor h[ mu* ( ! rock-salt b$s on ! e>? t has be5 precipitat$ out ( ocean wat]4 ,x is pla91 !re=e1 t ! rate ( any process t is controll$ by s _m conditions _c be us$ 7ev5 mak+ g5]\s all[ances = irregul>ities & inaccessible data7 z a reliable guide to evaluate geologic "t4 8,x is a clock10 says ,h>k]1 8:i* n[ hurries & n[ creeps or /&s /ill1 & x _c be tru/$ z a timekeep]40 ,any e/imate bas$ on ! temp]ature ( ! e>?1 or ( ! sun1 enc\nt]s simil> practical difficulties1 = ! temp]ature ( a body may n be con/ant4 ,x may rise or x may fall4 ,fur!r1 ! rate ( *ange ( temp]ature is controll$ by a v>iety ( conditions1 su* z ! am\nt ( en]gy radiat$1 ! supply ( en]gy & s =?4 ,nor is !re any record ( ! imm5se quantity ( heat produc$ by igne\s ag5cies & radio-activity4 ,ano!r e/imate1 "o ( ! e>lie/1 0 bas$ on ! rate ( life trans=mation 9 successive p]iods4 ,! geological s]ies 7 divid$ into twelve p]iods & x 0 believ$ t #bj1jjj1jjj ye>s 7 requir$ = an entire *ange 9 ! species ( ea* p]iod1 or #bdj1jjj1jjj ye>s 9 all4 ,?is does n include ! "t 9 :i* we h no record ( plant or animal life4 ,!re is no reasonable debate z to ! passage ( "o species to ano!r4 ,x is cle>ly manife/$ 9 ! succession ( fauna f\nd today all ov] ! _w 9 ! s$im5t>y rocks4 ,ev5 ! mo/ casual /ud5t ( paleontology is conv9c$ ( ?is gl>+ tru?4 8,! brutal cog5cy ( a slab ( fossils c\ld be hat$ & f\ght1 b c\ld n be ga9said40 ,b :5 we are confront$ ) ! "q ( sett+ a /&>d ( measur+ geologic "t by means ( ?is paleontological record1 m precisely1 ?r\gh ?is biological process1 we _c help pond]+ ov] ! grave unc]ta9ty ( ! result4 ,:5 we fix our gaze ^u a trilobite1 a ?ree-lob$1 crab-l creature 7,pl4 ,,ii fig4 #a7 t rul$ ! seas 9 ! dim days ( ! ,cambrian p]iod 7p4 #aa7 & see t x 0 equipp$ ) gills & swimm+ organs1 ) p[]s ( dige/ion & excretion1 ) specific organs ( circulation & reproduction & ) motor & s5sory n]ves1 & comp>e x ) "o ( its tribe1 a pres5t day horse%oe crab1 7,pl4 ,,ii fig4 #b7 we d n f9d any noticeable progress 9 /ructure1 9 intricacy or 9 ! degree ( specialization4 ,yet ! "t t has elaps$ s9ce ! ,cambrian is1 accord+ to a mod]ate e/imate1 ne>ly #fjj1jjj1jjj ye>s6 7p4 #aa74 ,geologic record te/ifies t evolution awaits environm5tal *ange1 t animals 9 "s way adju/ !mselves to _! environm5t1 ei!r by disc>d+ or modify+ old *>act]s or by acquir+ new ones4 ,yet1 :at are kn[n z 8immortal0 types1 su* z ! bra*iopods1 ,l+ula1 ,crania & ,t]ebratula 7,pl4 ,,iii figs4 #a-c7 or ! pelecypods1 su* z ,pect51 ,p9na & ,arca 7,pl4 ,,iii figs4 #d-f7 or ! ga/ropods1 su* z ,pleurotom>ia1 ,natica & ,tro*us 7,pl4 ,,iii figs4 #g-i71 h )/ood all possible environm5tal *anges & h /eadfa/ly held _! own ev] s9ce we h records ( _! v e>ly appe>ance on e>?4 ,on ! contr>y1 we h records ( types t h yield$ s rapidly to *ange t _! evolution is almo/ explosive4 ,x is almo/ incompreh5sible h[1 )9 su* a limit$ p]iod ( "t1 fi%es h *ang$ into amphibians1 amphibians into reptiles1 & reptiles into birds & mammals 7,pl4 ,,iv figs4 #a-d74 ,) ^! conflict+ evid5ces />+ u 9 ! face1 ) ! k t ! entire organic _w has be5 subject to e>?-wide p]iods ( long /agnation & rapid int5sive *ange1 "o may well pond] :e!r x is )9 our p[] to e/abli% a /&>d = measur+ geologic "t on ! evid5ce ( life trans=mation4 ,! /udy ( ! succession ( faunas--! *ange ( "o species to ano!r1 c only indicate ! magnitude ( "t involv$4 ,x _c af=d any basis1 :atsoev]1 = a concrete expression ( geologic "t4 ,dur+ ! la/ ?ree decades1 a numb] ( radio-active *anges ( "o *emical elem5t into ano!r h be5 discov]$ & /udies ( c]ta9 m9]als & rocks conta9+ v>i\s radio-active elem5ts h creat$ means to calculate _! age ) rem>kable accuracy4 8,a /udy ( ! v>i\s radio-active elem5ts conta9$ 9 m9]als & rocks10 says ,h>k]1 8has %[n t x is possible1 9 c]ta9 favorable cases1 to calculate directly _! age 9 ye>s40 ,! radio-active m9]als are commonly f\nd 9 igne\s rocks4 ,!y are widely di/ribut$ all ov] ! _w4 ,! p>5ts ( ! :ole s]ies ( radio-active elem5ts are uranium & ?orium4 ,!y possess ! highe/ atomic weights ( all kn[n elem5ts4 ,ea* ( ^! p>5tal elem5ts trans=ms itself ?r\gh a succession ( *anges4 ,! f9al product ( uranium is ! =mation ( ! metal lead & ! gas helium4 ,^! trans=mations take place 9 "o direction only1 t is1 f an elem5t ( high] atomic weight to an elem5t ( l[] atomic weight4 ,x has also be5 demon/rat$ beyond "q t ^! trans=mations are unalt]able by any process :atsoev] & t !y are indep5d5t ( temp]ature1 pressure or any o!r physical or *emical /ate4 ,temp]atures up to #b1ejj ,c4 & pressures up to #fjj tons p] squ>e in* h n be5 f\nd to influ5ce ! rate ( trans=mation4 ,"t e/imat$ on ! basis ( ^! processes1 !re=e1 (f]s a m reliable result ?an t obta9$ by any o!r me?od hi!rto kn[n4 ,detail$ descriptions ( h[ ! metal uranium sl[ly & regul>ly breaks d[n 9 a desc5d+ s]ies into ! metal lead & ! gas helium1 w be f\nd 9 ! lit]ature on radio-activity4 ,= our purpose1 x suffices to say t accord+ to ,b>rell1 an atom ( uranium :i* breaks up w ultimately give rise z a /able product to eight atoms ( helium & "o atom ( lead4 ,s9ce ! rate ( trans=mation is kn[n1 data = calculat+ ! age ( ! m9]al & ) x ! rock =mation ( :i* x is a "p1 c be obta9$ by measur+ ! quantity ( helium & lead 9 ! rock & comp>+ x ) ! quantity ( uranium 9 ! same volume ( mat]ial4 ,b1 z helium is a gas1 x is likely t a c]ta9 portion ( x leaks out & consequ5tly ! e/imate ( age on ! basis ( h[ long helium _h be5 9 contact ) uranium & lead is to be reg>d$ z a m9imum e/imate4 ,= example1 ! age ( ! m9]al ?orianite t occurs abundantly 9 ! s&s & gravels ( ,ceylon has be5 e/imat$ to be #bhj1jjj1jjj ye>s1 b ! m9]al is d\btless mu* old]1 z1 ev] s9ce x 0 brok5 away f its orig9al home 9 ! pegmatite dikes ( ,ceylon1 x lay expos$ to ! action ( wea!r+ & x 01 !re=e1 v likely t dur+ all ^! ye>s a c]ta9 p]c5tage ( its helium cont5ts _h leak$ away4 ,b e/imates bas$ on ! lead ratios ( radio-active m9]als (f] results consi/5t among !mselves4 ,t is1 :5ev] fre%1 prim>y1 uranium-be>+ m9]als ( ! same geological age h be5 exam9$1 ! lead ratios are always f\nd to rema9 con/ant4 ,! value ( ! ratios increases or decreases z ! geological age ( ! respective m9]al increases or decreases4 ,9 o!r words1 ! lead ratios are 9 keep+ ) ! geological age4 ,! proc$ure ( apply+ ! lead ratio 9 calculat+ geological "t c be briefly /at$ ?us3 ,! rate ( production ( lead f uranium c be readily calculat$4 ,! rate at :i* helium is g5]at$ is accurately kn[n & ! quantity ( lead lib]at$ 9 ! same "t is approximately #f4e times t ( helium4 ,9 a ye> "o gram ( uranium produces #a4be #aj grams ( lead1 & at ?is rate #h1jjjb million ye>s w be requir$ = ! production ( "o gram ( lead4 #b ,a m rec5t & accurate computation r$uces ?is #h1jjj million ye>s to #g1ejj million ye>s4 ,!re is no s]i\s difficulty 9 apply+ ?is me?od = measur+ geologic "t1 except t x is necess>y to det]m9e :e!r ! lead is ( radio-active orig9 or orig9al lead4 ,! pres5ce ( orig9al lead is likely to m> ! con/ancy ( ! lead ratio ess5tial = accurate results4 ,b ord9>y lead ne$ n be confus$ ) uranium lead1 z ! atomic weight ( ord9>y lead is #bjg4a & t ( uranium lead is #bjf4b4 ,values betwe5 ^! two figures imply a mixture ( two types ( lead4 ,= reliable calculations1 a s]ies ( fre%1 prim>y m9]als ( ! same geological age %[+ a con/ant lead ratio ( atomic weight #bjf4b ne$s to be exam9$4 ,! foll[+ table %[s ! geologic "t t has elaps$ betwe5 ! fir/ evid5ces ( life & ! pres5t1 z calculat$ by ,b>rell f radio-active data4 ,! figures are 8 m9imum & maximum e/imates4 ,x w be notic$ f ! figures 9 ! table t ! e>lie/ life ( :i* we h fossil records is ab\t #a1ejj1jjj1jjj ye>s old4 ,f ?is1 x c\ld be safely conclud$ t ! inception ( life on e>? mu/ h tak5 place mu* e>li]4 ,x is q significant t ea* geological era1 occasionally a geological p]iod1 has its *>act]i/ic gr\p+ ( life develop$ f ! life ( prec$+ p]iods4 ,z we climb high] 9 ! geological column1 life becomes m & m complex & specializ$4 ,f ! "o-cell$ life ( ! ,ar*eozoic x passes ?r\gh ! inv]tebrates--fi%es--amphibians--reptile s--birds & mammals to man ( ! ,rec5t "t4 ,s9ce ,b>rell's publication ( ! e/imates ( geologic "t z measur$ by means ( radio-activity1 "s fur!r /udies h be5 made along ! same l9e1 b1 z no g5]ally accept$ results h %[n any m>k$ diff]5ces f ,b>rell's results1 x has be5 deem$ advisable to use 8 age data z p]haps our pres5t mo/ adequate guide z to ! l5g? 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