264 
INTERNAL STRUCTURE AND 
ished to two hundred, one hundred and fifty, one 
hundred, fifty feet, and so on, till nearest the 
margin it becomes almost inappreciable. Sec- 
ondly, the rate of motion is not the same through- 
out the length of the glacier, the advance being 
greatest about half-way down in the region of the 
neve , and diminishing in rapidity both above and 
below ; thus the onward motion in the higher 
portion of a glacier may not exceed twenty to 
fifty feet a year, while it reaches its maximum 
of some two hundred and fifty feet annually in 
the neve region, and is retarded again toward the 
lower extremity, where it is reduced to about 
one fourth of its maximum rate. Thirdly, the 
glacier moves at different rates throughout the 
thickness of its mass ; toward the lower extrem- 
ity of the glacier the bottom is retarded, and the 
surface portion moves faster, while in the upper 
region the bottom seems to advance more rapidly. 
I say seems , because upon this latter point there 
are no positive measurements, and it is only in- 
ferred from general appearances, while the for- 
mer statement has been demonstrated by accu 
rate experiments. Remembering the form of the 
troughs in which the glaciers arise, that they 
have their source in expansive, open fields of 
snow and neve , and that these immense accumu- 
lations move gradually down into ever-narrowing 
channels, though at times widening again to con- 
