ESTIMATES OF TIME 823 



rate of denudatiou. It will ordinarily accumulate more slowly, as shown 

 by the maintenance of a baselevel surface of deposition, with its implica- 

 tion of the control of sedimentation by the slowness of subsidence. 



The mean rate of subsidence in a geosyncline has apparently been such 

 that a fairly large fraction of the mechanical sediment has been caught 

 in it. This is shown by the frequent passage of shales into limestones 

 within or beyond the bounds of the geosynclines. On the other hand, at 

 the times of greatest diastrophism and continental emergence an unknown 

 quantity of waste has been carried to the margins of the continental ter- 

 race and lost to observation. We may regard, then, as safe, a conclusion 

 that the Paleozoic era was several hundred million years in length. 



^Notwithstanding the inexact nature of the total of geologic time as 

 determined by the study of erosion and sedimentation, this line of attack 

 will be valuable for estimating the relative duration of the different 

 periods, the times and the totals being checked by some other and more 

 exact method. 



A few geologists have perceived the untenable nature of the assumption 

 that denudation and sedimentation have progressed in the past at the 

 present rate. Some have not published their views specifically. Those 

 who have indicated this opinion in published articles which have come to 

 the writer's attention are E. C. Andrews,^^ A. Holmes,^^ and A. Harker.^^ 



In conclusion should be mentioned Goodchild's estimate, based on a 

 detailed discussion of the stratigraphic sequence. He obtained 704,000,- 

 000 years since the beginning of the Cambrian, 93,000,000 being given 

 to the Tertiary, 236,000,000 to the Mesozoic, 375,000,000 to the Paleo- 

 zoic.®^ He reached these large figures by taking the rate of deposition 

 of sandstone as one foot in 1,500 years ; shales, one foot in 3,000 years ; 

 limestones, one foot in 25,000 years. He also considered the signi|icance 

 of unconformities. Goodchild, however, did not take into consideration 

 many possible factors, such as the rhythmic nature of sedimentation and 

 its ultimate control by subsidence ; so that his estimate doubtless appeared 

 to differ from others by a mere preference for the longer periods of time 

 and in apparent confiict with the clear evidence of the rapid accumula- 

 tion of individual beds. There is, however, much of value in his detailed 

 discussion, and his results, published before the first estimates based on 

 radioactivity, are in notable accord with those reached in the present study. 



80 E. C. Andrews : Erosion and its significance. Journal and Pi-oc. of the Roy. Soc. of 

 New South Vi^ales, vol. xlv, p. 1.32 ; read Aug. 2, 1911. 



"1 Arthur Holmes : The age of the earth. London and New York, Harper Bros., 1913. 



«2 A. Harker : Geology in relation to the exact sciences, with an excursus on geological 

 time. Nature, March 2.5, 191.5. 



«■'' .T. G. Goodchild : Vice-president's address, Nov., 1896. Proc. Royal I'hysical Society 

 of PMinburgh, vol. xiii, part 3, 1897, pp. 259-308. 



