There is another element in the movement of the soils which, though less appreciable, is still of great importance. The agents of decay which produce and remove the detritus, the chemical changes of the bed rock, and the mechanical action which roots apply to them, along with the solutional processes, are constantly lowering the surface of the mass. In this way we can often prove that a soil continuously existing has worked downward through many thousand feet of strata. In this process of downgoing the country on which the layer rests may have greatly changed its form, but the deposit, under favourable conditions, may continue to retain some trace of the materials which it derived from beds which have long since disappeared, their position having been far up in the spaces now occupied by the air. Where the slopes are steep and streams abound, we rarely find detritus which belonged in rock more than a hundred feet above the present surface of the soil. Where, however, as on those isolated table-lands or buttes which abound in certain portions of the Mississippi Valley, as well as in many other countries, we find a patch of soil lying on a nearly level surface, which for geologic ages has not felt the effect of streams, we may discover, commingled in the _débris_, the harder wreckage derived from the decay of a thousand feet or more of vanished strata.
When we consider the effect of organic life on the processes which go on in the soil, we first note the large fact that the development of all land vegetation depends upon the existence of this detritus--in a word, on the slow movement of the decaying rocky matter from the point where it is disrupted to its field of rest in the depths of the sea. The plants take their food from the portion of this rocky waste which is brought into solution by the waters which penetrate the mass. On the plants the animals feed, and so this vast assemblage of organisms is maintained. Not only does the land life maintain itself on the soil, and give much to the sea, but it serves in various ways to protect this detrital coating from too rapid destruction, and to improve its quality. To see the nature of this work we should visit a region where primeval forests still lie upon the slopes of a hilly region. In the body of such a wood we find next the surface a coating of decayed vegetable matter, made up of the falling leaves, bark, branches, and trunks which are constantly descending to the earth. Ordinarily, this layer is a foot or more in thickness; at the top it is almost altogether composed of vegetable matter; at the bottom it verges into the true soil. An important effect of this decayed vegetation is to restrain the movement of the surface water. Even in the heaviest rains, provided the mass be not frozen, the water is taken into it and delivered in the manner of springs to the larger streams. We can better note the measure of this effect by observing the difference in the ground covered by this primeval forest and that which we find near by which has been converted into tilled fields. With the same degree of rapidity in the flow, the distinct stream channels on the tilled ground are likely to be from twenty to a hundred times in length what they are on the forest bed. The result is that while the brook which drains the forested area maintains a tolerably constant flow of clean water, the other from the tilled ground courses only in times of heavy rain, and then is heavily charged with mud. In the virgin conditions of the soil the downwear is very slow; in its artificial state this wearing goes on so rapidly that the sloping fields are likely to be worn to below the soil level in a few score years.