10 



THE PRINCIPLES OF SOIL MANAGEMENT 



These aggregates of minerals, or rocks, are essentially 

 without order or arrangement. The minerals are in 

 irregular crystals or fragments of greatly differing sizes 



closely packed to- 

 gether. The great 

 variety of miner- 

 als, as well as the 

 different physical 

 forms of the same 

 mineral, is pro- 

 ductive of an infi- 

 nite variety of 

 rocks. While in- 

 dividuals may dif- 

 fer greatly, there 

 is an easy and 

 gradual transition 

 from one form to 

 another which 

 renders it impos- 

 sible to draw hard 

 and well-defined lines separating each species of rock 

 from every other species. They blend one into the 

 other, not only in structure and crystalline form but 

 also in chemical composition. 



The classification of rocks is based upon these facts, 

 and they are grouped broadly under four main heads, 

 the distinctions being their origin and structure. Each 

 oi the main divisions is again divided into groups and 

 families, the distinctions being those of mineral and 

 chemical composition, structure and mode of occurrence. 



Fig. 2. Photomicrograph of diorite rock. Com- 

 pare with Figs. 3, 5, and 6, which have a different 

 mineral composition, crystalline form and struc- 

 ture. These differences determine the type and 

 rate of their weathering. (Lord.) 



