Agriculture 



325 



Soils Classified According to Origin 



Soils are classified according to their origin into series, and 

 into types as to texture and chemical characters. The rocks 

 of all formations disintegrate under the weathering action of 

 climatic influences, as heat and cold, wind and rain. Erosion 

 carries the soil, broken rock, from mountains, hills and slopes, 

 to lower lands. Broken rock becomes soil when it remains in 

 one place long enough for a definite cover to gather at the sur- 

 face where plants may gain a foothold. Thus broken rock 



Courtesy U. S. Bureau of Soils 



FIG. 98. Vineyard near Hollister, San Benito County, 

 loam. Diablo Range in background. 



Gravelly sandy 



becomes soil when it holds still long enough for vegetation to 

 become established. Sunshine and moisture combine to trans- 

 form the minerals of the rocks into plant food. Soils are classi- 

 fied into series on the basis of origin, color, topography, and 

 structural characteristics. Residual soils are derived from con- 

 solidated rocks by weathering, on hills and slopes. In the 

 northern part of the State soils derived from sedimentary rocks 

 form the Altamont series; soils from igneous rocks, the Lassen 



