GEOLOGIC TIME. 309 



10,000,000 years and a maximum estimate of 5,000,000,000,000 [5 thou- 

 sand million] years.* Mr. McGee, in speaking of these estimates, says: 



''These general estimates are indefinite, and the minima, mean, and 

 maxima are alike unworthy of final acceptance; but they stand for a 

 real probletn and not a merely ideal one, and represent actual condi- 

 tions of the known earth; and, so far as the science of geology is con- 

 cerned, the maximum estimate is quite as probable as the minimum, 

 while the mean is much more probable than either." t 



Upham. — Prof. Warren Upham, after reviewing various estimates of 

 geologic time, concludes that the " probable length of Glacial and Post- 

 Glacial time together is 30,000 or 40,000 years, more or less ; but an 

 equal or considerably longer preceding time, while the areas that 

 became covered by ice were being uplifted to high altitudes, may per- 

 haps with good reason be also included in the Quaternary era, which 

 then would comprise some 100,000 years." He then applies Prof. 

 Dana's time ratios and concludes that the time needed for the earth's 

 stratified rocks and the unfolding of its jilant and animal life must be 

 about 100,000,000 years.f Mr. Upham's paper gives a number of illus- 

 trations of geologic phenomena from Tertiary and Pleistocene geology 

 that bear upon the time duration of these epochs. 



From the foregoing estimates of geologic time, the only conclusion 

 that can be drawn is that the earth is very old, and that nmn'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 

 exposed upon the North American continent. I soon found, however, 

 that the time at my disposal would make this impracticable, and I 

 decided to take up the history of the deposits that accumulated in 

 Paleozoic time on the western side of our continent, in an area that for 

 convenience I shall call the Cordilleran sea. This was (;hosen because 

 (1) I was personally acquainted with many of its typical sections, (2) 

 there was abroad and almost uninterrupted sedimentation during Pal- 

 eozoic time, and (3) there was a prosi)ect for obtaining more satisfac- 

 tory data as a basis of calculation, since calcareous deposits are in 

 excess of those of mechanical origin. 



We will now consider several points in relation to the growth or evo- 

 lution of the North American continent, as the dei)Osition of mechan- 

 ical sediments depend to a considerable extent on the character of the 

 adjoining land area, and chemical sedimentation is also infiuenced by it. 



GROWTH OF THE CONTINENT. 



The Algonkian sediments were deposited in interior and bordering 

 seas that tilled the dej^ressions and extended over the margins of the 



* Science, Juue 9, 1893, vol. xxi, p. 309. 



t Loc. cit., p. 310. 



t Am. Jour. ScL, 1893, vol. XLV, pp. 217, 218. 



