THE DRIP i AND GEOLOGIC TIME WAS 
have by it at the present time the data which enable us to form 
a definite minimum estimate of the time required for the depo- 
sition of the drift in North America, on the presumption that 
this was done through the agency of land-ice, or glaciers. If it 
is assumed that the drift was water laid, either altogether or to 
any considerable extent, altogether different elements enter into 
and affect the calculation, but that is not the assumption of the 
present paper. 
It is perhaps conceivable that the climatic conditions during 
the formation of the ice-sheet were such that it was deposited 
by precipitation simultaneously over the greater part of the 
area it occupied. The névé, in other words, might have been 
almost coterminous with the glacier, only a narrow external rim 
being excluded. The evidence, however, of motion throughout 
at least the greater part of its extent is afforded by the erratics 
many of which have traveled 600 or 700 miles or even more 
from their original beds. A bit of jasper conglomerate found 
south of Cincinnati must clearly have traveled from the north 
shore of Lake Huron, and fragments of Archean or eruptive 
rocks found abundantly along the southern limit of the drift in 
Illinois could have had no nearer source than northern Wisconsin 
500 or 600 miles away, if indeed they have not a still more north- 
ern origin. A bowlder or pebble from the north shore of Lake 
Superior, if found in southern Illinois, would have traveled nearly 
or quite 800 miles, and while Iam not sure that any such have 
been identified, their occurrence there is altogether within the 
bounds of probability. Such erratics, according to the glacier 
theory, must have been conveyed as subglacial or intraglacial 
detritus and must have progressed with the ice certainly at no 
greater rate than the ice itself and almost certainly at a much 
slower one. We have no certain evidence what the progress of 
the glacier was, but it could not have beena rapid one. In exist- 
ing glaciers the most rapid rate of motion is about seventy-five 
feet a day, but this occurs only during one or two months in 
summer, and in two or three exceptional Greenland glaciers 
where the ice,so to speak, is under pressure down a favorably 
