32 SOILS. 



rocks; potash feldspar (orthoclase) with quartz and mica forms 

 granite and gneiss; feldspars containing soda and lime (either 

 or both) form part of many other crystalline rocks, such as 

 basalt, diabase, diorite, gabbro and most lavas. The feldspars 

 are decomposed by weathering rather readily, and are import- 

 ant in being the chief source of clays as well as of potash in 

 soils. When acted upon by carbonated water, the bases 

 potash, soda, and lime or carbonates, the silica being mostly 

 displaced; while the silicate of alumina takes up water and 

 forms kaolinite, the essential basis of clays, and one of the 

 most important constituents of soils; imparting to them the 

 necessary firmness and cohesion, together with other important 

 physical properties, discussed more in detail hereafter. 



While thus on the one hand feldspars are the source of clay, 

 on the other they supply one of the most essential ingredients 

 of plant food, viz. potash ; which is first dissolved by the water 

 in the forms of carbonate and silicate, but in most cases soon 

 becomes fixed in the soil by forming more complex (zeolitic) 

 combinations. The soda not being retained by the soil as 

 strongly as is potash is washed through into the country drain- 

 age; while if lime is present, it mostly remains in the form of 

 the carbonate. 



Orthoclase feldspar contains nearly 17% of potash; 

 Leucite, a related mineral occurring in some lavas, contains 

 21.5%. The other feldspars contain only a few per cent, 

 sometimes none. 



Other silicate minerals, so far as they contain the same 

 bases, are acted upon similarly to the feldspars. 



In the decomposition of the feldspars by carbonated water, the com- 

 pounds of potash and soda so formed are soluble in water, those of 

 lime and magnesia are insoluble or nearly so. Hence pure clays can 

 be formed only in the decomposition of the potash- and soda-feldspars 

 (orthoclase, albite) while in the case of lime feldspar (labradorite) and 

 the mixed feldspars (plagioclase, anorthite) calcareous clays (marls) 

 are the result. Lime feldspar resists decomposition more tenaciously 

 than do those containing large proportions of the strong bases potash 

 and soda ; potash feldspar especially is attacked most readily, and is 

 the main source of the formation of the valuable deposits of porcelain 

 earth or kaolin, which is essentially a mixture of kaolinite with fine silex 

 and more or less of undecomposed feldspar, and is of a chalky texture. 



