CHAPTER III. 



THE MAJOR SOIL-FORMING MINERALS. 



Since the several stratified rocks, such as sandstones, 

 shales, claystones, clays, limestones, etc., are themselves 

 but the outcome of the same disintegrating and decom- 

 posing influences upon the crystalline rocks by which 

 soils are now formed, we must study the action of these influ- 

 ences upon the minerals composing- the latter rocks in order to 

 gain a comprehensive understanding of the subject. While 

 the number of different minerals known to science is very 

 large, such study need not go beyond a small number of the 

 chiefly important, rock-forming species which are so generally 

 distributed as to require consideration in this connection. 

 These minerals are the following : Quartz and its varieties ; 

 the several feldspars ; hornblende and augite ; the micas ; talc 

 and serpentine. Calcite, gypsum and dolomite, though not 

 contained in the older rocks, must be considered because of 

 their forming large rock deposits by themselves; and zeolites 

 require mention because, though rarely forming a large pro- 

 portion of rocks, they are of special importance as soil ingre- 

 dients. 



Quartz and the minerals allied to it consist essentially of 

 dioxid of silicon, usually without (quartz proper) but partly 

 also with water in combination (opal and its varieties). Sili- 

 con is next to oxygen the most abundant element found on the 

 earth's surface. It occurs largely in the various forms of 

 quartz, alone, or as one ingredient of compound rock-masses; 

 the rest, in combination (as silica) with various metallic oxids. 

 forms the important group of silicate minerals, constituting 

 the bulk of most rocks. 



Quartz occurs frequently in crystals (rock crvstal ; six-sided 

 prisms terminated by six-sided pyramids), clear or variously 

 colored ; but more abundantly as quartz rock or quartzite, read- 

 ily known by its hardness, so as to strike fire with steel, and 



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