218 W. Upham — Estimates of Geologic Time. 



areas that became covered by ice were being uplifted to high 

 altitudes, may perhaps with good reason be also included in 

 the Quaternary era, which then would comprise some 100,000 

 years. The best means for learning the relative lengths of 

 Tertiary and Quaternary time I think to be found in the 

 changes of faunas and floras since the beginning of the Ter- 

 tiary era, using especially the marine molluscan faunas as most 

 valuable for this comparison. Scarcely any species of marine 

 mollusks have become extinct or undergone important changes 

 during the Glacial and Recent periods, but since the Eocene 

 dawn of the Tertiary nearly all of these species have come into 

 existence. Judged upon this basis, the Tertiary era seems 

 probably fifty or a hundred times longer than the Ice age and 

 subsequent time ; in other words, it may well have lasted two 

 millions or even four millions of years. Taking the mean of 

 these numbers, or three million years, for Cenozoic time, or 

 the Tertiary and Quaternary ages together, we have precisely 

 the value of Professor Dana's ratios which he himself assumes 

 for conjectural illustration, namely, 48,000,0()( years since the 

 Cambrian period began. But the diversified types of animal 

 life in the earliest Cambrian faunas surely imply a long ante- 

 cedent time for their development, on the assumption that the 

 Creator worked before then as during the subsequent ages in 

 the evolution of all living creatures. According to these ratios, 

 therefore, the time needed for the deposition of the earth's 

 stratified rocks and the unfolding of its plant and animal life 

 must be about a hundred millions of years. 



Reviewing the several results of our different geologic esti- 

 mates and ratios supplied by Lyell, Dana, Wallace and Davis, 

 we are much impressed and convinced of their approximate 

 truth by their somewhat good agreement among themselves, 

 which seems as close as the nature of the problem would lead 

 us to expect, and by their all coming within the limit of 

 100,000,000 years which Sir William Thomson estimated on 

 physical grounds. This limit of probable geologic duration 

 seems therefore fully worthy to take the place of the once 

 almost unlimited assumptions of geologists and writers on the 

 evolution of life, that the time at their disposal has been prac- 

 tically infinite. J^o other more important conclusion in the 

 natural sciences, directly and indirectly modifying our concep- 

 tions in a thousand ways, has been reached during this century. 



The error by which Mr. McGee, in the estimate stated in 

 the early part of this article, wanders so far astray, consists in 

 his relying largely on Dr. Croll's theory for the cause of the 

 Glacial period, whereby he concludes that this period was of 

 great length and that the ice-sheets were due to astronomic 

 conditions while the land through the Ice age had somewhat 



