216 W. TPpham — Estimates of Geologic Time. 



From this wide range of concurrent but independent testi- 

 monies, we may accept it as practically demonstrated that the 

 ice-sheets disappeared from North America and Europe some 

 6,000 to 10,000 years ago. But having thus found the value 

 of one term in our ratios of geologic time divisions, we may 

 know them all approximately by its substitution. The two 

 inches assumed to represent the postglacial portion of the 

 Quaternary era may be called 8,000 years ; then, according to 

 the proportional estimates by Davis, the Triassic period was 

 probably 2,400,000 years ago ; the time since the Carbon- 

 iferous period has been about four or five millions of years ; 

 and since the middle of the Cambrian period, twice or perhaps 

 four times as long. Continuing this series still farther back, 

 the earliest Cambrian fossils may be 20 or 25 millions of years 

 old, and the beginning of life on our earth was not improbably 

 twice as long ago. 



Seeking to substitute our measure of postglacial time in 

 Professor Dana's ratios, we are met by the difficulty of ascer- 

 taining first its proportion to the p receding Glacial period, and 

 then the ratio which these two together bear to the Tertiary 

 era. It would fill a very large volume to rehearse all the 

 diverse opinions current among glacialists concerning the his- 

 tory of the Ice age, its wonderful climatic vicissitudes, and the 

 upward and downward movements of the lands which are 

 covered with the glacial drift. Many eminent glacialists, as 

 James Geikie, WahnschafFe, Penck, De Geer, Chamberlin, 

 Salisbury, Shaler, McGee, and others, believe that the Ice age 

 was complex, having two, three, or more, epochs of glaciation, 

 divided by long interglacial epochs of mild and temperate 

 climate when the ice-sheets were entirely or mainly melted 

 away. Professor Geikie, in a recent very able paper, + claims 

 five distinct glacial epochs, as indicated by fossiliferous beds 

 lying between deposits of till, and by other evidences of great 

 climatic changes. In this country Mr. McGee recognizes at 

 least three glacial epochs. The astronomic theory of Croll 

 attributes the accumulation of ice sheets to recurrent cycles 

 which bring the earth alternately into aphelion and perihelion 

 each 21,000 years during the periods of maximum eccentricity 

 of the earth's orbit. Its last period of this kind, as before 

 stated, was from about 240,000 to 80,000 years ago, allowing 

 room for seven or eight such cycles and alternations of glacial 

 and interglacial conditions. The supposed evidence of inter- 

 glacial epochs therefore gave to this theory a wide credence ; 

 but the recent determinations of the geologic brevity of the 

 time since the ice-sheets disappeared from North America 



* " On the Glacial Succession in Europe," Trans. Eoyal Society of Edinburgh, 

 1892, vol. xxxvii, pp. 121-149, with map. 



