THE MAJOR SOIL-FORMING MINERALS. 31 



destroyed by this process, so as to be light and chalky through- 

 out, and readily crushed in tillage. The change is the more 

 striking when, as frequently happens, the hornstone pebble is 

 traversed by small veins of crystalline quartz, which remain as 

 a skeleton. 



Solubility of Silica in Water. It is easily shown experimen- 

 tally that the compound of silica with water (hydrate) is under 

 certain conditions readily soluble not only in pure water, but 

 also in such as contains carbonic acid. It thus occurs in nearly 

 all spring and well waters ; some hot springs deposit large 

 masses of it (sinter) ; and geological evidence clearly demon- 

 strates that quartz veins have as rule been formed from water- 

 solutions of silica. 



That silica in its soluble form circulates freely in the soil 

 water, is abundantly evident from the large amounts of it 

 which are secreted on the outside of the stems of grasses, horse- 

 tail rushes and other plants, imparting a gritty roughness to 

 their outer surface. In the case of the giant bamboo grass of 

 Asia, the silica accumulated on the outside of the joints forms 

 a hard sheath of considerable thickness, known to commerce 

 as tabashir. 



That among the first products of rock decomposition we 

 often find small amounts of the silicates of the alkalies (potash 

 and soda) has already been mentioned. It cannot be doubted 

 that the same continues to be formed in soil containing the 

 proper minerals ; and there they also take part in the formation 

 of the easily decomposable hydrous silicates designated as 

 zeolites, which are largely instrumental in retaining the " re- 

 serve " of mineral plant-food in soils. 



SILICATE MINERALS. 



Silica occurs in nature combined with the oxids of most 

 metals, forming silicates ; but most abundantly with the earths 

 (lime, magnesia, alumina) and alkalies (potash and soda). 

 These compounds are the most important in soil formation; 

 and among them the following are the chief: 



The Feldspars, which may be defined as compounds of 

 silicates of potash, soda or lime (either or all) with silicate of 

 alumina. They are prominent ingredients of most crystalline 



