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on without interruption ever since those lands came into existence.
The disintegrating action of rain in temperate and high latitudes is greatly aided by frost, and the same is the case in the elevated tracts of more southern latitudes. Rain renders the superficial portions of rock more porous, and thus enables frost to act more effectually; while frost, by widening pores and fissures, affords readier ingress to meteoric water. Water freezing in soils and subsoils and in the interstitial pores and minute fissures of rocks forces the grains and particles asunder, and when thaw ensues the loosened material is ready to be carried away by rain or melting snow and subsequently, it may be, by wind. The same process takes place on a larger scale in the prizing open of joints and the rending asunder of rocks and rock-masses. Hence in Arctic regions and at high levels in temperate and southern latitudes the wholesale shattering of rocks has produced immense accumulations of angular ddbris. To such an extent has this action taken place, that in some countries the rocks are more or less completely buried in their own ruins. By-and-by so great do these accumulations become that frost is unable to get at the living rock. The loose fragments, however, under which it lies concealed, are themselves shattered, crumbled, and pulverised, until they are in a condition to be swept away by wind or melting snow. By this means the solid rock again comes within reach of the action of frost, and so the work of disruption and disintegration continues. The great heaps or "screes" of rock-rubbish which cloak the summits and slopes of our mountains, and gather thickly along the base of precipice and cliff, have been dislodged by frost and rolled down from above, their progress downward being often aided by torrential rains, melting snow, and the alternate freezing and thawing of the saturated ddbris itself.
Some reference has already been made to the indirect action of plants in the disintegration of rocks. The various humus acids, as we have seen, are powerful agents of chemical change. Without their aid rain-water would be a less effective worker. The living plants themselves, however, attack rocks, and by means of the acids in their roots dissolve out the mineral matters required by the organisms. Further, their roots penetrate the natural division-planes of rocks and wedge these asunder; and thus, by allowing freer percolation of water, they prepare the way for more rapid disintegration. Nor can we neglect the action of tunnelling and burrowing animals, some of which aid considerably in the work of destruction. There can be no doubt, for example, that worms, as Darwin has shown, play an important part in the formation of soil, which is simply rotted rock plus organic matter.
We see, then, that the disintegration and decomposition of rocks is a process everywhere being carried on—from the crests of the mountains down to the sea, and in every latitude under the sun. No exposed rock-surface escapes attack. In parched deserts as in well-watered regions, in the dreary barrens of the far north as in the sunny lands of the south, at lofty elevations as in low-lying plains, the work of rock-waste never ceases. Here it is insolation that is the most potent agent of destruction; there it is rain aided by humus and carbonic acids; or rain and frost combine their forces to shatter and pulverise the rocks. In latitudes where frost acts energetically, the most conspicuous proofs of rock-waste are the sheets and heaps of ddbris that are ever travelling down mountainslopes, or gathering at the base of cliff and precipice. In lower latitudes the most impressive evidence of disintegration is the great thickness attained by rotted rock in positions where it is not liable to be readily swept away by running water.
Hitherto we have been considering the superficial parts of rock, as these are affected by weathering. We are not to suppose, however, that the alteration of a rock ceases immediately underneath its crust. Rotted rock is not the only evidence of decay. In the case of felspathic rocks, it is found that some of the constituent minerals, more especially the felspars, usually show traces of decomposition at depths of many feet or even yards below the weathered superficial portions. It is hard, indeed, to get a specimen of any such rock from the bottom of our deepest quarries which is perfectly fresh. Water soaks through interstitial fissures and pores, and finds its way by joints and other division-planes, so that chemical action, with resultant rock-decay, is carried on at the greatest depths to which water can penetrate. This underground water eventually comes to the surface again through similar joints, etc., opening upwards, and thus forms natural springs. All these springs contain mineral matter, derived from the chemical decomposition and solution of rock-constituents. Many, indeed, are so highly impregnated, that as soon as they are exposed to evaporation they begin to deposit some of their mineral matter. Thus vast quantities of rock-material are brought up from the bowels of the earth. To such an extent is this the case in certain regions, that the ground is undermined and the surface not infrequently subsides. In countries where calcareous rocks largely predominate, acidulated water filtering down from the surface through fissures and other division-planes has often licked out a complicated series of tortuous tunnels and galleries. So far has this process been carried on in some regions that the whole rainfall finds its way into subterranean courses, and the entire drainage of the land is conducted underground. The dimensions attained by many well-known limestone caverns, and the great width and depth of the channels through which subterranean rivers reach the sea, help us to appreciate the amount of rock-material which underground water is capable of removing. When we add to this all the mineral matter leached out at the surface and carried away by streams and rivers, it is obvious that in course of time the land cannot fail to have been considerably modified by chemical action alone. In point of fact, it can be shown that from the surface of certain regions hundreds of feet of various calcareous rocks have thus been gradually removed; while in other cases the contour of the ground has been notably affected by the collapse of underground channels and chambers. But if the results of the chemical action of meteoric water be most evident in places where calcareous rocks predominate, yet the thickness attained in other countries by the crusts of less soluble rocks shows plainly enough that the whole land-surface of the globe is affected by the same action.
We may now consider the mechanical action of terrestrial water, by means of which the more or less insoluble residue of disintegrated rock is removed. Weathered rock is generally very porous, and is thus readily pulverised by frost. Some crusts crumble away as they are formed, while others adhere more persistently. On slopes and in mountain-regions generally, decomposed and disintegrated materials are seldom allowed to remain long in situ—rain and melting snow soon sweep away the finer portions. Great thicknesses of rotted rock are, therefore, somewhat exceptional in such places. Where, on the other hand, the land-surface is plain-like, or gently undulating, and the drainage sluggish, weathered materials are not so readily removed. Nevertheless, under the influence of rain alone, or of rain and melting snow, the products of rock-waste are everywhere travelling,