26

THE GLACIERS.

rain; the block, like an umbrella, protects it from both ; its elevationmeasures the level of the glacier at a former period, and as the de-pression of surface is very rapid—amounting even to a foot per week,during the warm months of summer—the ice, like the fields, putsforth its mushrooms, which expand under the influence of the warmshowers, until the cap, becoming too heavy for the stalk, or the centreof gravity of the block ceasing to be supported, the slab begins to slide,and, falling on the surface of the glacier, it defends a new space ofice, and forthwith begins to mount afresh. These appearances arecalled Glacier Tables. Their origin was perfectly explained by DeSaussure.

Where sand derived from the moraines has been washed by super-ficial water-runs into the deep cavities which are occasionally formed inthe glacier, the accumulation is at length sufficient to check the pro-gress of the waste of ice, and what was a hole filled with sand, becomesa pyramid projecting above its surface, and coated with the protectinglayer.* These produce glacier cones, which are amongst the most sin-gular and apparently unaccountable of this class of phenomena. Theyare sometimes astonishingly regular, 20 or 30 feet in height, and 80 or100 in circumference ;—but this is one of the rarer appearances.

From what has been said, it will appear that a glacier has a remark-able tendency to reverse its contour , or to present at one time the mouldor cast of what it was at another; any part of the surface prominentlyexposed is sure to be speedily reduced, and the hollows, whether holesor cracks or water-runs, by being silted up are protected from fartherdecay. The valleys are literally exalted, and the hills levelled. It isowing to this beautiful compensation that the glacier maintains atolerable evenness of surface.

A converse action, however, may be noticed. It is always on asmall scale, and there are two causes. The first occurs from the col-lection of small objects of a dark colour and in no great quantity on thesurface of the ice, which absorbing the solar heat, transmit it quickly tothe ice beneath, and such particles of black sand, or even leaves which

See Agassiz, yjtudcs, chap. x.