182 THE LURE OF THE LAND 



SANDY AND CLAY SOILS. 



z< Hilgard has called attention to the fact that soils 

 formed with a small supply of water as in the arid 

 regions have a distinctive sandy character, while those 

 formed under the influence of an abundant supply of 

 water contain more clay. In the decay of rocks, there- 

 fore, a dry climate has a distinctly retarding effect on 

 the kaolinization of feldspathic rocks. The soils 

 formed in situ on the Atlantic border are therefore 

 chiefly clay loams, while on the Pacific border they 

 contain a larger quantity of sand. 



" The preliminary condition to the growth of vegeta- 

 tion is the formation of soil, but once started, vegeta- 

 tion aids greatly in the decomposition of rocks. Some 

 forms of vegetation, as the lichens, have apparently 

 the faculty of growing on the bare surface of rocks, 

 but the higher orders of plants require at least a little 

 soil. Vegetation acts as a rock disintegrant by shading 

 the surface and thus rendering the action of water 

 more effective, by mechanically separating the rock 

 particles by means of its penetrating roots and by the 

 positive action of the root juices. The rootlets of 

 plants in contact with limestone or marble dissolve 

 large portions of these substances, and while their ac- 

 tion on more refractory rocks is slower, it must be of 

 considerable importance. It is evident that the sol- 

 vent action of the acids of living plants is confined 

 almost exclusively to the particles of rocks proximate 

 to points of exudation. The organic matter intro- 

 duced into the soil by vegetation also promotes decay 

 still further, both directly and by the formation of 

 acids of the humic series. This matter also furnishes 

 a considerable portion of carbon dioxid which is 



