FIG. 162.—A large glacial bowlder split by a growing tree near East Lansing, Michigan (after a photograph by Bertha Thompson).] =The rock mantle and its shield in the mat of vegetation.=—Through the action of weathering, the rocks, as we have seen, lose their integrity within a surface layer, which, though it may be as much as a hundred feet or more in thickness, must still be accounted a mere film above the underlying bed rock. The mechanical agents of the breakdown operate only within a few feet of the surface, and the agents of rock decomposition, derived as they are from the atmosphere, become inert before they have descended to any considerable depth. The surface layer of incoherent rock is usually referred to as the _rock mantle_ (Fig. 163). Where the rock mantle is relatively deep, as it is in the states south of the Ohio in the eastern United States, there is found, deep below the outer layer of soil, a partially decomposed and disintegrated rock, of which the unaltered minerals lie unchanged in position but separated by the new minerals which have resulted from the breakdown of their more susceptible associates. While thus in a certain sense possessing the original structure, this altered material is essentially incoherent and easily succumbs to attack by the pick and spade, so that it is only at considerably greater depths that the unaltered rock is encountered. [Illustration: FIG. 163.—Rock mantle consisting of broken rock, above which is soil and a vegetable mat. Coast of California (after a photograph by Fairbanks).] Because of the tendency of mantle rock to creep down upon slopes it is generally found thicker upon the crests and at the bases of hills and thinnest upon their slopes (Fig. 164). In the transformation of the upper portion of the mantle rock into soil, additional chemical processes to those of weathering are carried through by the agency of earthworms, bacteria, and other organisms, and by the action of humus and other acids derived from the decomposition of vegetation. The bacteria particularly play a part in the formation of carbonates, as they do also in changing the nitrogen of the air into nitrates which become available as plant food. Within the humid tropical regions ants and other insects enter as a large factor in rock decomposition, as they do also in producing not unimportant surface irregularities. [Illustration: FIG. 164.—Diagram to show the varying thickness of mantle rock upon the different portions of a hill surface (after Chamberlin and Salisbury).]