integrated as normal granites and basalts. Their joints, moreover, are not only less uniform, but frequently very abundant and closely set. Such rocks, therefore, are readily broken up. Mountains carved out of them usually show sharp crests and peaks, while their slopes are hidden under curtains of angular débris, through which ever and anon are protruded reefs, ridges, buttresses, and bastions of such portions of the rock-mass as are less profusely jointed. (See Fig. 78, p. 203.)

In short, we may say that every well-marked rocktype breaks up and weathers in its own way, so that under the influence of denudation each assumes a particular character. We see this even in the case of well-bedded aqueous rocks. Planes of bedding and jointing no doubt are the lines of weakness along which rocks most readily yield, but each individual rock-species weathers after its own fashion-the different kinds of shale, sandstone, conglomerate, and limestone are decomposed, disintegrated, and crumbled down at different rates, and each in a special way, according to its mineralogical composition and state of aggregation. Thus, although a region built up of bedded aqueous rocks may show the same general configuration throughout-horizontal strata giving rise to pyramidal-shaped hills and mountains, while inclined strata of variable consistency present us usually with a series of escarpments and dip-slopes -yet with all this sameness the details of rock-sculpturing may be singularly varied. And the same is

FIG. 78. VIEW OF MESA VERDE AND THE SIERRA EL LATE, COLORADO. (Hayden's Report for 1875.) Table-land in foreground built up of horizontal sandstones and shales (Cretaceous). Mountains in background consist of crystalline 203 igneous rocks (trachytes).

true of the crystalline schists. Mountains composed of such rocks have much the same general configuration. But when viewed in detail they show with every change in the character of the rock some corresponding change in the aspect of the surface. Again, in the case of granite, gabbro, and other massive igneous rocks, all these doubtless break up and produce characteristic configurations. But in each individual case we may note many details of sculpturing which are not the result of jointing, but of variations in the texture, and even in the mineralogical composition of the rock. We may note further that one and the same kind of rock does not necessarily always present quite the same aspect under weathering and erosion. Much will depend on the character of the climate, on the elevation of the region in which it occurs, and on the nature of the surface, whether, for example, that be steeply or gently inclined.

The characteristic forms assumed by rocks are, of course, best seen in places where these are well exposed. In low-lying tracts the rock-surface is usually more or less concealed beneath alluvial deposits and other superficial accumulations of epigene action. It is in river-ravines and along the sea-coast, or better still amongst the mountains, that rock-weathering must be studied. Even at the higher levels, however, the rocks are often largely concealed under their own ruins. Sheets and cones of débris extend downwards from the base of every projecting cliff and buttress. Hence in the case of mountains carved out of bedded