12 



THE PRINCIPLES OF SOIL MANAGEMENT 



5. Igneous, aqueous, aeolean and metamorphic 

 rocks. The igneous rocks are parent to all the other 

 forms. They may be arranged according to the amount 

 of silica they contain, those that are rich in that com- 

 pound being termed acid, and those that are lean, 

 basic. In this order, some of the most abundant rock 



types are granite, 

 quartz, syenites, 

 diorites, gabbro, 

 diabase and ba- 

 salts. 



Of the aqueous 

 rocks the chemical 

 precipitates are 

 relatively of small 

 importance. They 

 seldom form ex- 

 tensive rock 

 masses and are 

 usually intimately 

 mingled with 

 other types of 

 rock, especially those of the sedimentary group. The 

 most important ones agriculturally are the sulfates, 

 represented by gypsum beds. Certain phosphatic deposits 

 and some chlorides also belong in this group. 



The aqueous sedimentary rocks are the most import- 

 ant agriculturally of any of the groups of rock, and 

 especially of the aqueous rocks, because of their large 

 surface distribution and their physiography. They are 

 composed of the fragments derived from the degenera- 



Fig. 4. Photomicrograph of fossiliferous lime- 

 stone. (Lord.) 



