4 8 



SCIENCE AND SOIL 



silicate. Silicates from which water can be separated are called 

 hydrated silicates or, sometimes, acid silicates: 



Steatite (soapstone) Mg 6 Si 4 O 13 (OH) 2 . 



Kaolin (clay) Al 2 Si 2 O 5 (OH) 4 . 



Commonly, the silicates of the earth's crust are more or less 

 mixed, so that samples of pure compounds are rarely, if ever, 

 found in native state. Following are the results of analysis of 

 specimens of orthoclase, kaolin, and steatite as found in nature : 



TABLE 3. PERCENTAGE COMPOSITION OF SILICATES 



While the samples of orthoclase and kaolin were fairly pure, the 

 steatite contained other metals aggregating almost as much as 

 the magnesium. 



Granite and gneiss. These are among the most common rocks, 

 the former being of igneous or eruptive origin, while gneiss is 

 essentially the same material in sedimentary stratified form. 

 In other words, when granite has been disintegrated by the action 

 of heat and cold, rain and frost, has been transported by wind and 

 flowing water, has been redeposited in strata over river bottoms, or 

 ocean beds, and has become reformed into compact masses by the 

 cementing action of acids, alkalies, or salts, it is then called gneiss. 

 Gneiss is one of the oldest stratified rocks, and was formed chiefly 

 previous to the beginning of plant or animal life on the earth. 



Granite and gneiss consist principally of the four mineral groups, 

 felspar, hornblende, mica, and quartz. Of these the felspar group 



