188 GEOLOGY AND CHEMISTRY OF SOILS. 



dominating so as to give the rock a slaty and shining appear- 

 ance. 



4. The argillaceous slate and clay slate are made up prin- 

 cipally of quartz and alumina, or argillite, \thich seems to 

 be decomposed feldspar, containing from 7 to 10 per cent, of 

 potash. 



5. Talcose slates consist of talc, alumina and quartz. 



6. Hornblende rocks and hornblende slate are composed 

 mostly of hornblende. 



7. Graywacke is formed of quartz, clay slate and lime. 



8. The trappean rocks have a similar constitution. 



9. Limestones generally contain clay, feldspar, porphyry 

 and clay slate, although there are extensive beds of the pure 

 carbonate of lime. 



10. The various sandstones and slates are composed mostly 

 of silex and alumina, and hence are formed of the minerals 

 quartz and feldspar. 



Sect. 5. Origin of Soils, 



Having attended to the manner in which the simple bodies 

 are united to form the rocks, the way is now prepared to 

 describe the process by which the rocks are converted into 

 soils. 



The researches of modern geologists have established the 

 fact, that all soils were originally formed by the disintegra- 

 tion, decomposition and wearing away of rocks. The rock 

 has been gradually pulverized, and brought into the condi- 

 tion of soil. This effect has been produced by the mechani- 

 cal and chemical agency of air, water, living and decaying 

 vegetables. This process is constantly going forward. 



1. The oxygen of the atmosphere combines chemically with 

 the metals and decomposable minerals, and, by forming new 

 compounds, causes them to crumble down. Water also im- 

 parts its oxygen, and produces a similar effect. The surface 



