Glacial Geology in America. — Fair c J did. 171 
larly Leverett, Coleman and Hershey, has so strongly con- 
firmed the view of distinct ice invasions separated by intervals 
of deglaciation that this has passed quite beyond the point of 
argumentation, and the question now is as to the extent and 
duration of the stages of deglaciation, or the relative length 
and importance of the glacial and interglacial time divisions. 
In 1894, professor Chamberlin presented the whole mat- 
ter, in condensed form, in Geikie's "The Great Ice Age," and 
set the example, subsequently followed by Geikie, of denoting 
the several glacial time divisions by geographic names, using 
the names of areas where the individual deposits are typical. 
In 1895 he gave similar names to the interglacial epochs. The 
contemporaneity in Europe and America of the climatic oscil- 
lations and the epeirogenic movements, at least in a general 
way, seems quite demonstrated by the studies of Chamberliii 
and Geikie. 
Time Subsequent. — A question of great popular interest 
concerning glaciation is that of time, — the time in years, espe- 
cially since the ice disappeared. Glacialists will in honesty 
have to admit that they cannot yet fully satisfy that proper 
curiosity. That the time is very brief, judged by geologic time 
standards, since the ice sheet finally disappeared from our re- 
gion, seems certain. Judging from the freshness of the glacial 
scorings and the deposits, 5,000 or 10,000 or 15,000 years is 
thought by many glacialists to be a fair estimate of the length 
of their exposure. But no reliable chronometer has. yet been 
found. Niagara gorge has been regarded as the best time- 
piece, but fuller study has made the "Niagara Problem" more 
complex and uncertain. It seems likely that the volume of 
Niagara's water has varied greatly for long periods, as the 
history of the river is intimately connected with that of the 
glacial lakes of the upper lake basins. It is admitted that 
during the ice retreat the Laurentian basin was lower, as re- 
gards sea level, than at present; and was progressively lower 
northward and northeastward. It also seems certain that at 
the time of the ice removal the waters found lower outlets 
eastward from lake Huron than by way of lake Erie, and that 
such outflow persisted until the slow differential uplift raised 
the northern outlet above the St. Clair outlet. Hence for one, 
or probably two, long stages Niagara river carried only the 
