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 pelogen- 
ous, 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. 
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- 
