40 NATURE AND PROPEETIES OF SOILS 



gneiss ^ are generally clayey in nature, although loams and 

 even stony loams may occur if the limestone was sandy or 

 cherty and if the igneous rocks carried much quartz. Di)lo- 

 mites weather more slowly than limestone and often give 

 rise to gravelly and stony types. Sandstone of course pro- 

 duces sandy soils, although a soil from an argillaceous sand- 



FiG. 8. — Diagram showing the relationship between the underlying rocks 

 and the overlying residual soils. Gettysburg, Pa. (After Emerson.) 



stone may be rather heavy. Quartzite and slaty soils are 

 generally shallow, and unfavorable, both in texture and fer- 

 tility, for crop growth. Soils from basic igneous rocks, such 

 as diorite and basalt, generally produce sticky reddish or yel- 

 lowish clays containing little quartz. Rocks that carry con- 

 siderable mica, such as schists, give rise to highly micaceous 

 soils. 



^ For a complete discussion of the influence of various parent rocks on 

 the resultant residual soil see Emerson, H. L., Agricultural Geology, 

 Chap. IV J Kew York, 1920. 



