Tin' Flow of Glaciers. — Uphant. 27 
beautifully laminated ice strata, but to the flow of the ice 
fields under the pressure of the overlying ice and firn. Tin- 
superincumbent weight in all these examples of the produc- 
tion of glacier lamination may have been equal, at least, to 
the weight with which the Alpine glaciers are pressed in pass- 
ing the bends and constrictions of their valleys or ;it the base 
of their steep and crevassed frozen cascades. 
Relation to Drift Erosion, Transportation, and Deposition. 
With the general onward current of the glacier, there is 
probably, as the writer thinks, a component of motion by 
drag from the friction-hindered ice of the valley sides toward 
the freer flowing center, as indicated by the courses both of 
the marginal crevasses, trending up stream into the glacier, 
and of the shear planes of blue and white veining, which 
trend across the crevasses and pass down stream into the 
swifter central current. The arched cross section, however 
forbids the marginal moraines from being swept away from 
the sides; and this part of the lateral motion, most important 
close to the rock walls of the valley, is not traceable far out 
into the glacier, which there receives abundant inflow from 
the main descending ice current. 
Similarly the bed of the glacier, in being worn away by the 
rasping boulders, gravel, and sand frozen in the bottom of 
the ice. has glacial currents which, by the drag of the freer 
flowing higher part of the ice stream, are borne gradually up 
from the rock or drift upon which the glacier lies, carrying 
their drift along shear planes and in the general body of the 
ice to bights that J have elsewhere estimated to be a quarter 
or a third of the whole thickness of the glacier or ice-sheet. 
In regions now receiving abundant snowfall, as Greenland 
and the Antarctic continent, little ablation takes place on the 
border of the outflowing ice, and its englacial drift there 
maintains nearly the same ratio of its altitude, in comparison 
with the whole thickness of the ice, as in the much deeper 
part, where also the englacial drift doubtless reaches far 
higher, at distances from 50 to 200 miles or more back from 
the boundary. Hut in a region like Alaska, having less snow 
than formerly and consequent gradual decrease of the ice 
borders by ablation, much englacial drift becomes superglacial 
covering the attenuated ice margin. 
