24 SOILS: PROPERTIES AND MANAGEMENT 



space and the greater the encouragement to physical 

 agencies. As physical changes open the way for chemical 

 decay, coarse texture will ultimately encourage decomposi- 

 tion as well as disintegration. 



Lastly, the disruptive forces of the rock will be in- 

 fluenced by the chemical composition of the minerals 

 and the mineral composition of the rock. A rock made 

 up of minerals that offer but little resistance to decay 

 will naturally reduce readily and quickly to a soil. 

 Rocks that very largely bear minerals which are re- 

 fractory in their nature, however, may never decompose 

 far enough or rapidly enough to give a soil of any agri- 

 cultural significance. The next step, then, in the study 

 of soil formation is a consideration of the relative resistance 

 of the minerals and the rocks. 



22. The law of mineral and rock decay. — Considerable 

 work has been done on the comparative solubility of 

 minerals both in pure and carbonated water, but in most 

 cases it has proved somewhat inconclusive. Neverthe- 

 less we are able, by consulting the work of Miiller, 1 Clark, 2 

 Daubree, 3 and others, to arrange some of the commoner 

 minerals in the order of their solubility, the most resistant 

 minerals heading the list : — 



1. Quartz 6. Epidote 11. Apatite 



2. Muscovite 7. Serpentine 12. Olivine 



3. Biotite 8. Talc 13. Calcite 



4. Orthoclase 9. Hornblende 



5. Plagioclase 10. Augite 



1 Miiller, R. Solution of Rocks in Carbonated Water. Jahrb. 

 k-k Geol. Reichsanstalt, Vol. XXVII, p. 25. 1877. 



2 Clark, F. W. Data of Geochemistry. U. S. Geol. Survey, 

 Bui. 330, p. 401. 1908. 



3 Daubree, A. Solubility of Orthoclase. Etudes de Geol. 

 Experimentale, pp. 27 and 252. Paris, 1847. 



