6 4 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Other conditions of cooling may give rise to a compact, stony 

 mass, composed of minute crystals (stony rock) or to a rock 

 containing large crystals of one or more kinds of mineral em- 

 bedded in a stone or glass matrix (prophyry). These differences 



Fig. 14. Microscopic appearance of porphyry. 



Fig. 15. Microscopic appearance of granite. 



in the structure of rocks of the same composition will give rise 

 to different soils. Glassy rocks will produce more or less homo- 

 genous particles, while crystalline rocks will weather into par- 

 ticles of different kinds, perhaps composed of the different 

 minerals. 



