78 THE HABITAT 



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 decom- 

 pose ; 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 pelogew- 

 ous, clay-producing, psammogenous, sand-forming, or pelopsammogenoiis, 

 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. 



114. The structure of soils. The water capacity of a soil is a direct 

 result of the fineness of the particles. Since the water is held as a thin 

 surface film by each particle or group of them, it follows that the amount 

 of water increases with the water-holding surface. ,The latter increases as 

 the particles become finer and more numerous, and thus produce a greater 

 aggregate surface. The upward and downward movements of water in the 

 soil are likewise in immediate connection with the size of particles. The 

 upward or capillary movement increases as the particles become finer, thus 

 making the irregular capillary spaces between them smaller, and magnify- 

 ing the pull exerted. On the contrary, the downward movement of gravita- 

 tion water, i. e., percolation, is retarded by a decrease in the size of the soil 

 grains and hastened by an increase. Hence, the two properties, capillarity 

 and porosity, are direct expressions of the structure of the soil, i. e., of its 

 texture or fineness. Capillarity, however, increases the water-content of the 

 upper layers permeated by the roots of the plant, while porosity decreases 

 it. On the basis of these properties alone, soils would fall into two groups, 

 capillary soils and porous soils, the former fine-grained and of high water- 

 content, the- latter coarse-grained and with relatively little water. A third 

 factor, however, of great importance must be taken into account. This is 

 the pull exerted upon each water film by the soil particle itself. This pull ap- 



