« PreviousContinue »
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 ddbris extend downwards from the base of every projecting cliff and buttress. Hence in the case of mountains carved out of bedded rocks, the rectangular outlines tend to become obscured, projecting rock-ledges gradually disappear under piles of ddbris, and a smooth slope may replace in whole or in part the rectangular corbel-steps of the typical pyramid, while steep escarpments may be smoothed off to more or less gentle inclines. In the case of mountains composed of schistose rocks the general steep inclination and contorted character of the bedding and the varied character of the rocks themselves favour the preservation of abrupt and irregular slopes. There is a general absence of horizontal or gently inclined platforms upon which ddbris may come to rest. The great mass of the material loosened and detached by weathering rolls and shoots downwards to the screes accumulating at the base of the mountains. These, as denudation advances, are of course continually extending upwards. But the characteristic configuration of the rocks above the scree-line is maintained, and not obscured, as so frequently happens in the case of horizontal or gently inclined strata. Amorphous igneous masses break up in so diverse a manner, that mountains composed of such often show much variety of feature. The upper limits of the scree-line are very tortuous, here sweeping up almost to the very crest of a mountain, there hugging the base of gaunt cliffs and precipices. Or, when horizontal jointing is well defined, we may have a succession of abrupt ledges breaking the continuity of a scree-slope. When, on the other hand, vertical joints are most pronounced bare