Extract

Throughout the wide domain of geological inquiry, there is, perhaps, no subject of which it is easier to gain some idea, and yet at the same time more difficult to acquire an adequate conception, than Denudation. We all know how rains and frosts and chemical decomposition are employed unceasingly in modifying the aspect of hills and plains—how rivers are ever deepening and widening the valleys in which they flow—how the sea by its constant wave-action, aided by frosts and other agencies, tends to reduce to its own level the solid lands, with all their infinite variety of outline. It is no less generally known how the geological structure of a country must always influence its configuration—how the soft rocks will generally lie in the valleys, and those of a more durable nature occupy the heights—how the contour of the heights will vary according as these are formed of schists, of granite, of dolerite, or quartz-rock, or of any other well-marked species—and how this peculiar character is due to the unequal manner in which these various masses yield to the touch of the atmospheric forces:—so that a trained eye shall oftentimes detect, at a considerable distance, the nature of the rocks by reference to the contour of the hills alone. But while all are willing to admit that the subaerial agents may do something towards modifying the surface of a country, yet many will not allow that its straths and valleys have been mainly formed by these seemingly feeble forces. Nor

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