with Geographic Botany. — Soil. 



523 



wet season. The resulting materials are gradually deposited 

 on reaching the lower lands, and constitute the basis of soil. 

 In this manner have rivers ever been, and are still, the most 

 active agents in originating soil, whilst vegetation subse- 

 quently enriches it. 



Even after a clear acquaintance is gained with the mineral 

 composition of soils, and of the different organic substances 

 which also contribute to their formation, other circumstances 

 require to be taken into consideration before our knowledge 

 is complete ; for though these are extraneous, or form no part 

 of its physical characters, they have an influence over it as a 

 source of fertility. However good the soil, an unsuitable sub- 

 stratum may deprive it of nearly all its good qualities : a good 

 substratum should retain or dismiss moisture according to the 

 constitution of the incumbent soil. Inclined surfaces allow 

 the finer and more soluble substances to gravitate towards 

 the base, and are generally a good deal drained of moisture. 

 Plains have usually good and extensive soils, particularly any 

 valleys which may descend from them, and indeed valleys in 

 general : these may be regarded likely to have been formerly 

 the bottoms of lakes or other large bodies of water, in which 

 case an important portion of carbonaceous matter is mixed up 

 with the soil. 



The state of cohesion, or the condition of the aggregation v 

 of the particles, has its effects on vegetation, since the roots of 

 plants have a variety of different forms, which require some 

 adaptation of the soil to their organization. Argillaceous soils 

 are too consistent for many roots to penetrate, and plants 

 growing on them are fixed more on the surface than in the 

 soil. Sandy soils, though easily penetrated by the roots, are 

 so moveable, that the latter experience many chances of ex- 

 posure ; thus there are many large tracts of sand on the sur- 

 face of the globe which do not support the smallest vestige of 

 vegetation. Where sandy soils are not liable to be swept away, 

 and are moderately supplied with moisture, they support a 

 very tolerable vegetation. Solid rocks resist any attempts to 

 penetrate them according to their structure ; quartz rocks 

 strongly oppose the roots of plants as well as those w hich are 

 granular or crystalline ; schistose rocks and the various sand- 

 stones are more easily disintegrated ; and marl and chalk, 

 though not particularly favourable, support a moderate vege- 

 tation. 



Soils possess different capacities for retaining moisture ; and 

 th is is a highly important property, since a very great share 

 of their nutritive qualities is dependent on it. Aluminous 

 and argillaceous soils surpass all others in the quantity of 



2 M 2 



