16 



AGRONOMY 



A cubic mile of ordinary river water has been estimated to 

 contain about a quarter of a million tons of rock constituents. 

 It is not difficult to understand, then, that in many soils formed 

 by the decomposition of limestone rock, a layer of limestone 

 nearly a hundred feet thick may have been removed for every 



Photograph by II. L. Ilollister Land Co. 



FIG. 4. Twin Falls, Idaho 



Here the Snake River has worn a channel hundreds of feet deep in 

 hard igneous rock 



foot of soil left. In contrast to this, water, under certain con- 

 ditions, instead of decreasing the bulk of the soil in weathering, 

 may actually increase it by entering in combination with some 

 of the elements in the rocks. Thus granitic rocks in turning 

 to arable soil may be increased in bulk more than 80 per cent. 

 Weathering by disintegration. By disintegration the rocks 

 are broken up into small pieces like the original rock and 

 suffer little, if any, chemical change. In this process heat, 



