But in other parts of our own country we find proofs of denudation on a much grander scale than this; for example, in North Wales there are rocks now lying exposed at the surface which are of a very much greater antiquity than any that may be seen in the Wealden area, belonging to the very ancient periods known as the Cambrian and Silurian. These have evidently been exposed for a much longer time to the action of denuding forces; and the Welsh hills, as we now see them, are but fragments of what they once were. After carefully mapping out the rocks in the neighbourhood of Snowdon, noting their thickness, the directions in which they slope, or "dip," so that the structure of this region might be ascertained, as in the case of the Weald, it was found, on drawing sections of the rocks there, and putting in dotted lines to continue the curves and slopes of the strata as known at or near the surface, that from fifteen thousand to twenty thousand feet of solid rock must have been removed (see diagrams, chapter ix., p. 307). Applying the same method to the Lake District, it has been calculated that the amount of denudation which that beautiful country has suffered may be represented by twenty-six thousand feet. Turning to the other side of the Atlantic, we find the American geologists estimate that a thickness of five miles has been removed from a large part of the Appalachian chain of mountains (near their east coast), and that at least one mile has been eroded from the entire region between the Rocky and Wahsatch Mountains (see chapter ix.). In conclusion, we must bear in mind that mountains, in spite of the enormous erosion they have suffered, are more capable of resisting the ever active agents of denudation than the softer rocks that form the plains and lowlands, and consequently stand out in bold relief from other features of the earth's surface. This truth has been beautifully expressed in the following passage:--