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

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

rocks, the rectangular outlines tend to become obscured, projecting rock-ledges gradually disappear under piles of débris, 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 débris 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

rock-walls and steep ridges rise more or less abruptly above the limits of the depressed scree-line below.

In regions subject to well-marked dry and rainy periods even low grounds and plateaux not infrequently show much bare rock. This is due to the fact that disintegrated rock-material tends to be swept rapidly downwards by heavy torrential rains. Should the land be well clothed with vegetation, the reduction of the surface is much retarded. The rocks may become rotted to great depths, as in Brazil, but the decomposed material remains in situ. Where vegetable life in such latitudes is less prolific the surface becomes scorched and dried, and disintegrated rockmaterial is readily removed when the rainy season comes round. Under these conditions the surfacefeatures, due to epigene action, are usually strongly pronounced. A plateau of granitoid rock, for example, owing to inequalities of structure, texture, and composition, often yields a highly diversified surface; rounded blocks and boulders of all shapes and sizes appear scattered broadcast, while sporadic masses, stacks, cones, tors, crags, and peaks, and irregular winding gullies and depressions, are everywhere encountered. But the same phenomena, if somewhat less prominently developed, are seen again and again in temperate latitudes. The "tors" of Cornwall are in their way as striking as the kopjes of Mashonaland. Many other kinds of rock, after long exposure to the weather, present similar fantastic outlines. The "Quadersandstein" of Saxon Switzer

land, for example, which over considerable areas lies in approximately horizontal strata, has suffered great erosion, the characteristic features of the region being conical hills or pyramids and broad bastions, along the flanks of which the naked rock appears. Thus exposed to weathering, the sandstones yield along the vertical joint-planes and fall away somewhat unequally, and so stacks and columns eventually become separated from the main rock-masses, and often weather into odd and picturesque forms.

The surface-features assumed by limestone are very characteristic, and these, as in the case of all stratified rocks, are determined by bedding and jointing. But the soluble character of limestone causes it to weather in a manner peculiar to itself. Bare surfaces are eaten. into, and become irregularly honeycombed and furrowed the rock, in short, is corroded by the chemical action of rain. Should the ground be steeply inclined, the surface of the limestone shows numerous more or less parallel gutters and trenches, separated by narrow ridges which are frequently sharp and knife-edged. Upon gentler slopes the gutters are less regular, and the ridges are often somewhat rounded; the whole surface, indeed, may be rudely mammillated, and traversed or interrupted by abrupt furrows and smoother depressions. These appearances are most marked when the limestone is pure; when it contains much insoluble matter the characteristic ridges and trenches, rounded humps and hollows, are seldom well developed. It is needless to add that endless modifications