22 SOILS AND MANURES 



into the composition of granite and similar rocks, and is 

 very difficult to decompose. Albite, the soda felspar, 

 resembles it closely, but contains soda in place of potash. 

 Anorthite, the lime felspar, differs more widely ; it is 

 represented by the formula CaAl 2 Si20a, occurs chiefly 

 in the volcanic rocks and decomposes more readily. 



Nepheline. A silicate of soda and alumina, with some 

 potash ; replaces felspars in some lavas. 



Kaolin. A hydrated silicate of alumina, Si20 5 Al 2 (OH) 4 , 

 is produced along with potassium carbonate and 

 hydrated silica by the weathering of orthoclase. Pure and 

 dry, it forms an impalpable white powder which, when 

 mixed with water, is converted into a plastic mass. The 

 mixed water is easily evaporated ; at higher temperatures 

 the combined water is also driven off and does not reunite 

 with the residue. Kaolin is used in the manufacture of the 

 finer kinds of pottery and is called "china clay." It is not, 

 however, to be confused with ordinary agricultural clay. 

 The latter is often of very different nature, but usually 

 contains some kaolin, and owes its plasticity partly to the 

 presence of that substance. 



Zeolites. A large group of hydrated silicates of alumina 

 and alkalis or alkaline earths ; are so called from the froth- 

 ing or boiling appearance due to evolution of water when 

 they are strongly heated. They are usually soft and exhibit 

 a pearly lustre ; are probably derived from felspars or 

 nepheline, and apparently have been deposited from solu- 

 tion. They are found in the veins and cavities but do not 

 enter into the composition of rocks. The phenomena of 

 absorption have been attributed to the presence of these 

 minerals in soils. The formula CaAl 2 Si 6 Oi 6 + 6H a O 

 has been ascribed to sfcilbite, and NflaAlaSieOie + 4H 2 

 to natrolite. 



The potash-bearing silicates, such as mica, orthoclase, 

 etc., occur most plentifully in the granites and rocks of 



