350 The Glacial Theory of Prof. Agassiz. 
where it is soon frozen, and in freezing expands, according to a well 
known law. The upper strata, imbibing more water than the lower, 
dilate in a greater degree, but the lower strata, in dilating, carry the 
upper with them, and thus produce rents or crevasses. Again, the 
flanks of the glacier imbibe more water than the middle, and by their 
greater expansion give a curved form to the crevasses; and the lower 
end imbibes more water than the upper, in consequence of the more 
frequent rains and alternations of frost and thaw. Besides, as the upper 
end of the glacier, in expanding, pushes the rest before it, the accu- 
mulated effect of the whole expansion falls upon the lower end, which 
is found to travel quickest. The motion, too, is most rapid in summer, 
parts, the glacier creeps along slowly but surely. In 1827, M. Hugi 
constructed a hut on the glacier of the Aar, at the foot of a fixed rock . 
called Im Abschwung. It was found that the hut had receded 2200 feet 
from the fixed rock in 1836, and 4400 in 1840, showing that it had 
advanced about 250 feet per annum in the first nine years, and 550 in 
the four last. Taking summer and winter together, its motion had been 
about eight inches per day in the first period, and eighteen inches in 
the second. In glaciers which are much inclined, the motion is more 
rapid than this. 
Polished and Grooved Surface of Rocks.—The glacier in its course 
downward carries with it the fragments of rock, gravel, and sand which 
lie under it. These adhere to the ice, or are embedded in it, and as 
the mass glides slowly along, they abrade, groove, and polish the rock, 
and the larger masses are reciprocally grooved and polished by the 
rock on their lower sides. The effects of this abrasion on the bottom 
of the valleys may be conceived from the pressure applied. A cubic 
yard of sandstone weighs two tons, and if we assume the average 
ante of glacier ice to be two-thirds of that of common river ice, the 
upon each square yard of rock at the bottom of a glacier 100 
feet deep, will be equal to about sixteen tons, or the general pressure 
will be as great as would be produced by a bed of sandstone twenty 
four feet thick. Thus the various materials under the ice are pressed 
against the rock with am enormous force, while an equally great force 
of another kind, produced by the congelation of water, propels them 
downwards. ‘The sand, coxttng like emery, polishes the surface ; the 
pebbles, lik ratch and furrow it; and the large stones 
scoop out grooves in it. Portions of these substances, and of the rock 
too, are ground to the state of fine clay, and the whole of the movable 
matter, stones, pebbles, sand, and elay, are in course of time thrown 
out at the lower end of the glacier, where they form the terminal 
moraine. 
