204 W. UPHAM — PLEISTOCENE AND PRESENT ICE-SHEETS. 



puted, from various means of measurement and estimate, that the ice- 

 sheets were finally melted away some 6,000 to 10,000 years ago. 



Relation of the Ice Age to human and geologic History. — In Europe Pro- 

 fessor Geikie has shown that men had reached the neolithic stage of their 

 development before the ice-sheet of Denmark and Scandinavia vanished ; 

 and similarly in California neolithic implements, probably contempora- 

 neous with the Ice age, are found in gravels under the lava of Table 

 mountain. Other discoveries of stone implements have been made by 

 Dr Abbott and Professors Putnam and Shaler in the late glacial gravels 

 of Trenton, New Jersey ; by Mr J. B. Tyrrell, of the Canadian Geological 

 Survey, in a beach of the glacial lake Agassiz ; and by Mr McGee in the 

 sediment of the last great flood of the Pleistocene lake Lahoutan. 



The accumulation of the Pleistocene ice-sheets took place at or near 

 the beginning of man's existence, but was close to the present page in 

 the very long record of geology. The preglacial uplifting of the land 

 apparently occupied a far longer time than its glaciation, but both may 

 be well referred, as by Hilgard and Spencer, to the Quaternary era, which 

 also, according to Dana and Sir Archibald Geikie, should be considered 

 as extending to the present time. With this definition, the Quaternary 

 era may comprise 100,000 years, more or less. Judging from the com- 

 parative changes of marine molluscan faunas, the Tertiary era Avas prob- 

 ably twenty to forty times as long, comprising, therefore, some two to 

 four million years ; and according to Professor Dana's time ratios of this 

 and the earlier eras, the whole of the history of the earth's fossiliferous 

 rocks may be somewhere between thirty and sixty million years. This 

 estimate lies between that given for the earth's age from physical data 

 many years ago by Sir William Thomson (now Lord Kelvin), namely, 

 100,000,000 years, and that recently deduced by Mr Clarence King from 

 similar but more detailed investigations, which is 24,000,000 years. 



