204 The American <<<<>l<>(jist. April, imms 
of the modified drift is. theoretically, so relatively direct and 
simple, that faith in its sufficiency cannot be lightly relin- 
quished. 
In his recent paper on the "Discrimination of Glacial Accu- 
mulation and Invasion,"* Upham has, it seems t<> me, made a 
substantial contribution to the general theory of the Pleisto- 
cene ice-sheet; and my chief purpose now is to analyze this 
idea that the ice-sheet was, over a considerable part of the 
glaciated area, formed in situ by snow accumulation, trace it 
to its logical conclusions, and show that it also throws impor- 
tant light upon the more special problem of the englacial 
drift. 
Probable Early History of the Pleistocene Ice-sheet. 
In this discussion a sharp distinction may properly be made 
between mountainous tracts, like the Adirondacks, Green 
Mountains, and White Mountains, and the plain or peneplain 
surface characteristic of t he greater part of the glaciated area. 
The first-named topographic type, although the more excep- 
tional, may conveniently be considered first. 
In mountainous districts, or where the relied' features are so 
strongly accentuated as to cause appreciable climatic differ- 
entiation, the general refrigeration of the climate, due chiefly, 
it is probable, to a marked elevation of the northern part of 
the continent, led first to the development of glaciers of the 
alpine type in the higher valleys. These descended, under the 
influence of gravity, below the climatic zone in which they 
originated; as the level at which terminal ablation balanced 
the downward movement was gradually lowered, they became 
confluent; and finally, emerging from the mountains, they 
deployed upon the plain country, forming relatively sluggish 
or stagnant piedmont glaciers. But the extension of the pied- 
mont glaciers by simple invasion can not continue indefinitely 
for the reason that with progressive refrigeration the annual 
snowfall finally exceeds the annual melting over the plain 
country as well as the mountains: at first near the margins of 
the piedmont glaciers only, but gradually extending farther 
and farther from them. 
This cumulative snowfall, which mantled alike hills and 
valleys, and changed slowly through neve to glacial ice, must 
*Bull. Geol. Soc. of America, vi. 343-352. 
