=113. The origin of soils.= Rocks form soils in consequence of weathering, under the influence of physical and biotic factors. Weathering consists of two processes, disintegration, by which the rock is broken into component particles of various sizes, and decomposition, in which the rock or its fragments are resolved into minute particles in consequence of the chemical disaggregation of its minerals, or of some other chemical change. These processes are usually concomitant, although, as a rule, one is more evident than the other. The relation between them is dependent upon the character of the rock and the forces which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a rule disintegrate more rapidly than they decompose; sedimentary rocks, on the other hand, tend to decompose more rapidly than they disintegrate. In many cases the two processes go hand in hand. This difference is the basis for the distinction, first proposed by Thurmann, between those rocks which weather with difficulty and those which weather readily. The former were called dysgeogenous, the latter eugeogenous. Thurmann restricted the application of the first term to those rocks which produce little soil, but it seems more logical to apply dysgeogenous to those in which disintegration is markedly in excess of decomposition, and eugeogenous to those rocks that break down rather readily into fine soils. With respect to the general character of the soil formed, rocks are _pelogenous_, clay-producing, _psammogenous_, sand-forming, or _pelopsammogenous_, producing mixed clay and sand. The first two are divided into _perpelic_, _hemipelic_, _oligopelic_, _perpsammic_, etc., with reference to the readiness with which they are weathered, but this distinction is not a very practicable one. The grouping of soils into silicious, calcareous, argillaceous, etc., with reference to the chemical nature of the original rock, is of no value to the ecologist, apart from the general clue to the physical properties which it furnishes.