Erosion in high arid regions differs from that in regions of abundant rainfall in several ways. It is obvious that the valleys will develop more slowly in the former, that they will remain young longer, that the period necessary for the dissection of the surface is greater, that the watercourses will be less numerous, and that fewer of them will have permanent streams. There are certain other differences which are less obvious. If the arid region be high and composed of heterogeneous strata, the topography which erosion develops is more angular (Fig. 83) than that of the humid region. This is because there is less rock decay, and less vegetation to hold the products of decay. The more resistant beds of rock therefore come into greater prominence, especially on slopes, where they develop cliffs (Figs. 109 and 110). These general principles find abundant illustration in the plateaus of the western part of the United States,[48] where the cliffs are by no means confined to the immediate valleys of the streams (Fig. 1, Pl. XII). [Illustration: +Fig.+ 110.—A Butte. A characteristic feature of the arid plateau region of the West. (Dutton, Mono. II, U. S. Geol. Surv.)] EFFECTS OF UNEQUAL HARDNESS. In the preceding pages incidental reference has been made to the results of inequalities of rock resistance. This topic will now be considered more fully. =Rapids and falls.=—Returning for a moment to the hypothetical island with which our study of erosion began, let a horizontal layer of hard rock be assumed to run through it (H, Fig. 111). As the rain falls on the land and runs off over it, wear will be less rapid where the hard layer comes to the surface than at the higher or lower levels. As a result, the slope will become steeper at and below the outcrop of the hard layer, and less steep immediately above it, as shown by _ab_ in Fig. 111. Under these conditions the water passing over the hard ledge constitutes _rapids_. The increased erosion which accompanies the increased velocity makes the rapids more rapid. The process may continue until the water _falls_, rather than _flows_ over the hard layer (_cd_, Fig. 111). With continued rainfall the edges of the hard layer, together with the slopes above and below, would continue to recede toward the center of the island. Under conditions of absolute homogeneity of material, save for the hard layer specified, no valley would be developed, and therefore no stream.