Geologic Time — Walcott. 345 
this estimate he says: "It is not of course supposed that the calcula- 
tion here makes any approach to accuracy, but it is believed that it 
does indicate the order of magnitude of the time required.''* Dr. 
Alexander Winchell reduced geologic time still more in his estimate of 
3,000,000 years for the whole incrusted age of the world."!" Later writers, 
however, do not accept this, as we find Sir Archibald Geikie concluding 
on the basis of denudation and deposition that the sedimentary rocks 
would have required 73 millions of years tor their deposition, if denu- 
dation was at the rate of one foot in 730 years; or of 680 millions of 
years if at the slower rate of one foot in 0,800 years.J Mr. T. Mellard 
Reade adopted one foot in 3,000 years as the rate of average denudation 
throughout geologic time, and obtained a result of 95 millions of years 
as the time that has elapsed since the beginning of Cambrian time.§ 
M. A. de Lapparent is one of the few European continental geologists 
that has written on geologic time. On the basis of mechanical denu- 
dation and sedimentation he thinks that from 67 to 00 millions of 
years would suffice, at the present rate of sedimentation, for 
everything that has been produced since the consolidation of the 
crust.il The two most recent writers who have taken their initial 
datum point or "geochrone" from the consideration of late Cenozoic or 
Pleistocene phenomena have differed materially in their results. Mr. 
W J McGee estimated that the mean age of the earth is 15,000 million 
years, and that 7,000 million had elapsed since the beginning of Paleo- 
zoic time.** In a subsequent note he modifies this conclusion and gives 
as a mean estimate 6,000 million years, of which 2,400 million have 
elapsed since the beginning of the Paleozoic. This is based on a mini- 
mum estimate of the age of the earth of 10 million years and a maxi- 
mum estimate of five million million (5,000,000,000,000) years.tf Prof. 
Warren Upham concludes that Quaternary time comprises about 100,- 
000 years. He applies Prof. Dana's time ratio and finds on this basis 
that the time needed for the earth's stratified rocks and the unfolding 
of its plant and animal life must be about 100 millions of years. %% 
From the foregoing estimates of geologic time the only conclusion 
that can be drawn is that the earth is very old and that man's occupa- 
tion of it is but a day's span as compared with the eons that have 
elapsed since the first consolidation of the rocks with which the geol- 
ogist is acquainted. 
When I began the preparation of this paper it was my intention to 
carefully analyze the sedimentary rocks of the entire geologic series as 
*Island Life, 2d Ed., 1892, pp. 222-223. 
fWorld Life, or Comparative Geology. Chicago, 1888, p. :<7s. 
^Presidential Address; report of 62d meeting Hritisli Assoc. Adv. Sci., 1892, p. 21. 
^Measurement of Geological Time, Geol. Mag., vol. 10, 1893. pp. 99-100. 
:.De la mesare du temps par les plienomones de sedimentation. Hull. Soc. Geol. 
France, 3d ser., vol. 18. 1890, pp. 351-355. La Destinee de la terre ferine et duree des 
temps geoloeiques. Revue des questions scientifiques, July 1891. Pamphlet, Hrux- 
elles. pp. 1-158. 
♦♦American Anthropologist, vol. 5, 1892, p. 340. 
ttScience, vol. 21, 1898, p. 309. 
JiAm. Jour. Sci., vol. 15, 1893, pp. 217-21 8. 
