Page:A Treatise on Geology, volume 2.djvu/28

 14 winter, faster in a warm day than in a cold night, faster in some seasons than in others. Its motion is continual though unequal; faster in the middle than at the sides, and at the surface than in the deeper parts.

The daily motion at a point of the side of the glacier of Montanvert was found by Forbes to be 17.5 inches; and at the centre 27.1: the general proportion of the central to the lateral movement being 1375 to 1000.

In one year the average descent of the Mer de glace was found to be 563 feet. The velocities vary in different parts of the glacier. In the upper part of the glacier above Montanvert 674 feet in a year: lower down 479: at the "Angle," 770: and below Montanvert, 1000.

While thus moving downward, the glacier is subject to enormous waste by the action of sun and wind.

The waste of the glacier above Montanvert is thus given by Forbes:—

Thus only a small portion of the mass which quitted the snowy wilds at its source is found to reach the source of the Arve, which, indeed, is formed by a portion of that waste, which is thus indicated.

In the uppermost parts of the glacier some alternations appear of the more snowy and more icy parts of the mass; a kind of stratification. Farther down a peculiar structure, first distinctly described and explained by Professor J. Forbes, appears. This is the veined or ribboned structure, in which laminæ of blue compact ice alternate with other laminæ of ice full of air bubbles, placed in a vertical direction across the glacier. Lower down these plates are no longer vertical, but dip toward the source of the glacier; and they are no longer plane, but curved, so as to present a