of all the rotted material?-a question to answer which leads us into the very foundations of geological history.

C

Openings from the soil down into the rock underneath often afford instructive lessons regarding the decay of the surface of the land. Fig. 2, for instance, is a drawing of one of these sections, in which a gradual passage may be traced from solid sandstone (a) underneath up into broken-up sandstone (b), and thence into the earthy layer (c) that supports the vegetation of the surface. Traced from below upwards, the rock is found to become more and more broken and crumbling, with an increasing number of rootlets that strike freely through it in all directions, until it

a

FIG. 2.-Passage of sandstone upwards into soil.

passes insensibly into the uppermost dark layer of vegetable soil or humus. This dark layer owes its characteristic brown or black colour to the decaying remains of vegetation diffused through it. Again, granite in its unweathered state is a hard, compact, crystalline rock that may be quarried out in large solid blocks (a

FIG. 3.-Passage of granite upwards in Fig. 3), yet when traced upward to within a few feet

into soil.

from the surface it may be seen to have been split by innumerable rents into fragments which are nevertheless still lying in their original position. As these fragments are attacked by percolating moisture, their surfaces decay, leaving the still unweathered parts as rounded blocks (6), which might at first be mistaken for transported boulders. They are, however, parts of the rock broken up in place, and not fragments that have been carried from a distance. The little quartz veins that traverse the solid granite can be recognised running through the decayed and fresh parts alike. But besides being broken into pieces, the granite rots away and loses its cohesion. Some of the smaller pieces can be crumbled down between the fingers, and this decay increases towards the soil until the rock becomes a mere sand or sandy clay in which a few harder kernels are still left. Into this soft layer roots may descend from the surface and, like the sandstone, it passes upwards into the overlying soil (c).

Soil and Subsoil.-In such sections as the foregoing, three distinct layers can be recognised which merge into each other. At the bottom lies the rock, either undecayed or at least still fresh enough to show its true nature. Next comes the broken-up crumbling layer through which stray roots descend. This is known as the subsoil. At the top lies the dark band, crowded with rootlets and forming the true soil. These three layers obviously represent successive stages in the decay of the surface of the land. The soil is the layer of most complete decay. The subsoil is an intermediate band where the progress of decomposition has not advanced so far, while the shattered rock underneath shows the earlier stages of disintegration. Vegetation sends its roots and rootlets through the rotted rock. As the plants die, they are succeeded by others, and the rotted remains of their successive generations gradually darken the uppermost decomposed layer. Worms, insects, and larger animals that may die on the surface, likewise add their mouldering remains. And thus from animals and plants there is furnished to the soil that organic matter on which its fertility so much depends. The very decay of the vegetation helps to promote that of the underlying rock, for it supplies various organic acids ready to be absorbed by percolating rain-water, the power of which to decompose rocks is thereby increased (p. 32).

It is obvious, then, that in answer to the question, What becomes of the rotted material produced by weathering? we may confidently assert that, over surfaces of land protected by a cover of vegetation, this material in large measure accumulates where it is formed. Such accumulation will naturally take place chiefly on flat or gently inclined ground. Where the slope is steep, the decomposed layer will tend to travel down-hill by mere gravitation, and to be further impelled downward by descending rain-water.

If there is so intimate a connection between the soil at the surface and the rock underneath, we can readily understand that soils should vary from one district to another, according to the nature of the underlying rocks. Clays will produce clayey soil, sandstones, sandy soil, or, where these two kinds of rock occur together, they may give rise to sandy clay or loam. Hence, knowing what the underlying rock is, we may usually infer what must be the character of the overlying soil, or, from the nature of the soil, we may form an opinion respecting the quality of the rock that lies below.

But it will probably occur to the thoughtful observer that when once a covering of soil and subsoil has been formed over a level piece of ground, especially where there is also an overlying carpet of verdure, the process of decay should cease-the very layer of rotted material coming eventually to protect the rock from further disintegration. Undoubtedly, under these circumstances, weathering is reduced to its feeblest condition. But that it still continues will be evident from some considerations, the force of which will be better understood a few pages further on. If the process were wholly arrested, then in course of time plants growing on the surface would extract from the soil all the nutriment they could get out of it, and with the increasing impoverishment of the soil, they would dwindle away and finally die out, until perhaps only the simpler forms of vegetation would grow on the site. Something of this kind not improbably takes place where forests decay and are replaced by scrub and grass. But the long-continued growth of the same kind of plants upon a tract of land doubtless indicates that in some way the process of weathering is not entirely arrested, but that, as generation succeeds generation, the plants are still able to draw nutriment from fresh portions of decomposed rock. A cutting made through the soil and subsoil shows that roots force their way downward into the rock, which splits up and allows percolating water to soak downwards through it. The subsoil thus gradually eats its way into the solid rock below. Influences are at work also, whereby there is an imperceptible removal of material from the surface of the soil. Notable among these influences are rain, wind, and earth-worms. Wherever soil is bare of vegetation it is directly exposed to removal by rain and wind. Ground is seldom so flat that rain may not flow a little way along the surface before sinking underneath. In its flow, it carries off the finer particles of the soil. These may travel each time only a short way, but as the operation is repeated, they are in the course of years gradually moved down to lower ground or to some runnel or brook that sweeps them away seaward. Both on gentle and on steep slopes, this transporting power of rain is continually removing the upper layer of bared soil. Where soil is exposed to the sun, it is liable to be dried into mere dust, which is borne off by wind. How readily this may happen is often strikingly seen after dry weather in spring-time. The earth of ploughed fields becomes loose and powdery, and clouds of its finer particles are carried up into the air and transported to other farms, as gusts of wind sweep across. "March dust," which is a proverbial expression, may be remembered as an illustration of one way in which the upper parts of the soil are removed.

Even where a grassy covering protects the general surface, bare places may always be found whence this covering has been removed. Rabbits, moles, and other animals throw out soil from their burrows. Mice sometimes lay it bare by eating the pasture down to the roots. The common earthworms bring up to daylight in the course of a year an almost incredible quantity of it in their castings. Mr. Darwin estimated that this quantity is in some places not less than 10 tons per annum over an acre of ground. Only the finest particles of mould are swallowed by worms and conveyed by them to the surface, and it is precisely these which are most apt to be washed off by rain or to be dried and blown away as dust by the wind. Where it remains on the ground, the soil brought up by worms covers over stones and other objects lying there, which consequently seem to sink into the earth. The operation of these animals causes the materials of the soil to be thoroughly mixed. In tropical countries, the termite or "white ant" conveys a