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There are not a few kinds of rock other than those now referred to, but they may be neglected as, from our present point of view, of relatively little importance. Amongst them are rock-salt, gypsum, coal and lignite, ironstones, and other ores. All these, doubtless, are very notable and valuable, but they are neither so abundant nor so widely distributed as the above-described groups; in short, they occupy a very subordinate place in the architecture of the earth■s crust.
We have now to consider how the superficial or epigene agents attack and reduce rocks. And first, we may note that rocks at the surface are everywhere subject to changes of temperature—warmed by day and during summer, cooled at night and during winter. Thus they alternately expand and contract, and this tends to disintegration, for the materials of which they are composed often yield unequally to strain or tension. This is particularly the case with many crystalline felspathic rocks, such as coarse-grained granite, gneiss, and mica-schist—built up, as these are, of minerals that differ in colour, density, and expansibility. Even when a rock is homogeneous in composition, it is obvious that the heating and cooling of the surface must give rise to strain and tension. In countries where there is no great diurnal range of temperature, as in our own latitudes, any rock-changes due to this cause alone are hardly noticeable, since they are masked or obscured by the action of other and more potent agents. But in the rocky deserts of tropical and sub-tropical regions, bare of verdure and practically rainless, the effects produced by alternate heating and cooling are very marked. The rocks are cracked and shattered to a depth of several inches; the surfaces peel off, and are rapidly disintegrated and pulverised. Wind then catches up the loose material and sweeps it away, leaving fresh surfaces exposed to the destructive action of insolation. More than this, the grit, sand, and dust carried off by the wind are used as a sand-blast to attack and erode the rocks against which they strike. In this manner cliffs and projecting rocks are undermined, and masses give way and fall to the ground, where, subject to the same grinding action, especially towards the base, they eventually assume the appearance of irregular blocks supported upon pedestals. Mushroom-shaped rocks and hills of this kind are common in all desiccated rocky regions.
The transporting action of the wind, or "deflation," as it is termed, goes on without ceasing day and night and during all seasons; and the result is seen in the deeply eroded rocks, enormous masses of which, it can be shown, have been thus gradually removed. The evidence of denudation is conspicuous, but its products have for the most part been carried away. In some places, as Professor Walther remarks of the Libyan Desert, are great walls of granite rising to heights of 6000 feet, but showing no slopes of ddbris below, as would infallibly be present under temperate conditions of climate. In other places, again, are deeply excavated wadies containing no beds of gravel, grit, and sand, such as would not fail to show themselves had the depressions in question been formed by water-action alone. Everywhere, deep, cave-like hollows have been worn out in the rocks, and yet these hold no sediment or detritus, but are swept bare. The wind tends, in short, to transport all loose material from the scene of its origin to the borders of the desert.
In latitudes like our own, insolation doubtless shares in the disintegration of rocks, but the most conspicuous agent employed in that work is rain. Rain is not chemically pure, but always contains some proportion of oxygen and carbonic acid absorbed from the atmosphere; and after it reaches the ground organic acids are derived by it from the decaying vegetable and animal matter with which soils are more or less impregnated. Armed with such chemical agents, it attacks the various minerals of which rocks are composed, and thus, sooner or later, these minerals break up. The felspars and their ferro-magnesian associates, for example, are decomposed—the carbonic acid of the rain-water uniting with the alkalies and alkaline earths of those minerals to form carbonates, which are carried away in solution. The silica set free by this operation is also to some extent removed, while the insoluble silicate of alumina, or clay, remains behind. Such insoluble materials are frequently stained yellow-brown or red, owing to the pressure of ironoxides. In this way felspathic rocks gradually crumble down. Thus, granite, gneiss, basalt, and other rocks largely composed of felspar, usually show a weathered crust, which, according to the nature of the rock and the length of time its surface has been exposed, may vary from less than an inch up to many feet, or even yards, in thickness. Some granites, for example, are reduced to a kind of gritty clay which may be dug with a spade.
Argillaceous and silicious rocks are not so readily affected by the chemical action of rain. Not infrequently, however, when the grains of a sandstone are cemented together by some soluble substance, such as carbonate of lime, the rock will yield more or less readily to the solvent action of the water. All calcareous rocks, in short, tend to fall an easy prey. If they contain few or no impurities, they "weather" with little or no crust; the rock is simply dissolved. Limestones, however, are seldom quite so pure as this, but are usually impregnated in a greater or less degree with quartz, clay, or other substance, which after the carbonate of lime has been removed remains behind to form a crust. The red and brownish earths and clays that so frequently overlie calcareous rocks, such as chalk and limestone, are simply the insoluble residue of masses of rock, the soluble portions of which have been dissolved and carried away by surface-water.
In all regions where rain falls, the result of this chemical action is conspicuous; soluble rocks are everywhere dissolving, while partially soluble rocks are becoming rotten and disintegrated. In limestone areas it can be shown that sometimes hundreds of feet of rock have thus been gradually and silently removed from the surface of the land. And the great depth now and again attained by rotted rock testifies likewise to the destructive action of rain-water percolating from the surface. This is particularly noticeable in warm-temperate, sub-tropical, and tropical latitudes, where felspathic rocks are decomposed not infrequently to depths of a hundred feet and more. In temperate and northern regions, the amount of rotted rock is rarely so great. The thicker rockcrusts of southern latitudes are supposed to be due to the larger supplies of organic acids derived from the more abundant vegetation. To some extent this is probably true. But there is another reason for the relatively meagre development of rotted rock in temperate and northern regions generally. Those regions, as we shall learn later on, have recently been subjected to glacial conditions. Broad areas of temperate Europe and North America have been scraped bare by ice-sheets, resembling those of Greenland and the Antarctic Circle. In more southern latitudes, the rotted rocks have escaped such abrasion and denudation, and hence it is not strange that we should find them attaining so great a thickness. The decomposed rock-material encountered in the northern parts of Europe and America has been formed for the most part only since the disappearance of glacial conditions, while in southern regions rock-decay has gone