Agriculture 



329 



of wine. But wine-producing grapes do not grow on all kinds 

 of soil. Some thousands of acres of wine-grape vineyards are 

 on the "floor" soils of the Napa Valley. Certain varieties thrive 

 on the lower slopes of the mountains. The valley bottom soils 

 are the result of geologic processes that have been long going 

 on. The valley soils are composed of innumerable millions of 

 particles of broken rock granitic rocks, lavas, sedimentary 

 rocks. The soils have become "mature" under the action of 

 sun and moisture during long periods. From deposits of "raw" 

 rock fertile soils have been developed. They are "rich" because 

 they have not been leached of their soluble minerals. Chemi- 

 cally the minerals of the rocks have been released in the long 



Courtesy U. S. Bureau of Soils 



FIG. 100. Century-old pear trees, near San Juan Bautista, on Yolo silt 

 loam. These trees are the remnant of a pear-and-apple orchard planted 

 shortly after the founding of the San Juan Bautista Mission, in 1797. 



slow processes of the "maturing" of the soils from broken frag- 

 ments of rock. The soils have been formed from the "wash" 

 of the mountains surrounding during long ages. During the 

 long time since they were deposited the stone particles have 

 been acted upon by weathering agencies, and organic matter 



