Page:A Treatise on Geology, volume 2.djvu/32

 18 summer heat is greater. The lower limit of a glacier depends also on local climates, on the abundance of snow, the depth of the glacier, the slope of the valley, and the rapidity of downward motion: for as the glacier is subject to continual waste from atmospheric and terrestrial agency, the longer its course the more is it exposed to this waste. With a long course on a slight declivity glaciers cannot in general descend so far below the line of perpetual snow, as with a shorter course in a steeper glen. In the Alps the steep glaciers of Grindelwald and the valley of Chamouni descend to the level of 5,300 feet below the snow line, while that of the Aar, on an easier slope, reaches only to 2,650 feet. In Norway the glaciers descend 4,400 feet; but in the Pyrenees only 1,700 feet.

The average slope of the whole glacier from the Arveyron to the Col du geant is 8° 52′, and this is nearly the inclination of the upper part. In the middle part, terminating with Montanvert, it varies from 4° 19′ to 5° 5′, and below Montanvert grows so steep as to give measures of 12° and 20° 41′.

The slopes on which glacier movement is possible are of course somewhat less than those which are actually traversed by glaciers, because they are unequal. In some ingenious experiments made in temperatures which allowed the lower surface of a mass of ice to be just losing its solidity, Hopkins has found the following relations between the inclination of the surface on which motion takes place, and the velocity produced. Up to 12° the velocity is uniform.