Page:A Treatise on Geology, volume 2.djvu/25

 CHAP. VI. as if by the action of acidulated water, the origin of which, from the air or the neighbouring vegetable substances, is not hypothetical.

In no form is the moisture of the atmosphere inefficient in accelerating the disintegration of rocks. Collected in the joints and cavities of mountains, it loosens every thing by its expansion and relaxation; heaped into enormous glaciers on the summits and down the valleys of the Alps, it melts at its lower edges and on the lower surfaces, and thus is ever in motion downwards; augmented from above and diminished from below, its moving masses plough up the solid earth, and, by a wonderful and momentarily insensible energy, pile up, on each side of the icy valley, vast quantities of blocks of stone and heaps of earth, which slowly advance into the lower ground; and these sometimes bear trees and admit cultivation; till, in the course of changes which these rude climates experience, the whole is transported away by the river which flows beneath, and space is left for new augmentations from above. Perhaps no circumstances are so favourable to the collection of materials for rivers to sweep away, as the glacial crown and icy valleys of the Alps, accompanied by the thundering avalanche and frequent landslips, like those of the Rossberg and the Righi. What further happens to these materials belongs to the history of the river.

In modern geological theory, the glacier has become a power not less influential than in other days the diluvial wave; but it is a power in daily action, of which the laws are known and the effects measurable. If, in applying this power to earlier phenomena, we employ larger measures than nature now works by, or stretch our lines in directions where glaciers are now unknown, we are always amenable to the ascertained laws of glacial action. If we may now venture to say these laws are known, let geology gratefully own her obligation to the cultivators of physical science, who