242 The American Geologist. April, isns 
tioiis of forest beds between deposits of till, and of the peculiar 
drift accumulations named drumlins (as shown in the American 
GrEOLOOisT for December, 1892), both being attributable to stages 
in the general recession of the North American ice-sheet when 
increased snowfall and onflow of the ice slackened its retreat or 
caused it temporarilj' to re-advance. Under this view, the Ice 
age seems probably to have comprised only one great epoch of 
glaciation, with moderate oscillations of the ice-front, and to 
have been geologically brief. The length of the Post-glacial 
epoch, according to N. H. Winchell, Gilbert, Andrews, Wright, 
Prestwich, Mackintosh, and others, has been only about 0,000 to 
10,000 years. In Europe Prof. James Geikie has shown that 
men had reached the neolithic stage of their development before 
the ice-sheet of Denmark and Scandinavia had vanished; and 
similarly in California neolithic implements, probably contempo- 
raneous with the Tee age, are found in gravels under the lava of 
Table mountain. Other discoveries of stone implements, mostly 
of palaeolithic types, and of the flakes formed in their manufact- 
ure, have been made by Dr. Al)bott and Profs. Putnam and 
Shaler in the late glacial gravels of Trenton, N. J., by Mills and 
Metz in Ohio, by Miss Babbitt in Minnesota, by Tyrrell in a 
beach of the glacial lake Agassiz in Manitoba, and by McGee in 
the sediment of the last great flood of the Pleistocene lake 
Lahontan. Great elevation of the glaciated countries is believed 
by the author to have been the chief cause of the Ice age, and 
the pre-glacial uplifting probably occupied a longer tirce than the 
glaciation; l)ut 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 com- 
prise about 100,000 years. 
Mr. McGee, in discussion, doubted the reference of the ice 
accumulation to high altitude of the land, and spoke of re- 
cent studies by Mr. W. H. Holmes at Trenton, N. J., who 
supposes all the evidences of man's presence there to be post- 
glacial. 
Dr. A. R. C. Selwvn thought that the Pleistocene and present 
ce-sheets are most readily explained by changes in the course 
and volume of marine currents, and that glacialists Should take 
into account various concurrent causes for the glacial climate 
