28 THE MINERALS CONSTITUTING BOOKS 



corresponding to the formula H^MgaSiaO^ = silica, 44.1% ; mag- 

 nesia, 43.0%; and water, 12.9%. 



The prevailing color is green, though often spotted and 

 streaked; hence the name from the Latin serpentiniis^ serpent- 

 like. It has a somewhat greasy lustre and may be cut with a 

 knife, having a hardness of about 4 of the scale. The mineral 

 is always secondary, resulting mainly from the hydration of 

 magnesium or lime magnesium silicates. (See further on p. 107.) 



Glauconite. — This name is given to a somewhat variable 

 compound consisting essentially of silica, iron, alumina, and 

 water, with smaller amounts of potash, and incidentally lime, 

 magnesia and soda. The prevailing color is green, and as it 

 occurs in single granules or granular aggregates, it is com- 

 monly known as greensand. It is always a secondary mineral, 

 and has been formed and is still forming on many shallow sea- 

 bottoms which receive fine sediments derived from the breaking 

 down of siliceous crystalline rocks. (See under Greensand 

 Marl, p. 116.) 



The Zeolites. — Under this head are grouped a number of 

 minerals alike in being hydrous silicates of alumina with vary- 

 ing percentages of lime, potash, and soda. They are altogether 

 secondary minerals, resulting from chemical changes taking 

 place in pre-existing rocks, and indicate the first or deep-seated 

 stages of rock decay. In a more or less perfect condition they 

 have been assumed to occur in soils, having been derived from 

 the rocks, or, as is contended by some authorities, having formed 

 during the process of rock decomposition or in the soil itself. 

 It is thought possible that those constituents of a soil which 

 on analysis are found to be soluble, as the term is ordinarily 

 used, may, in part at least, have once existed as zeolites. Hence 

 their consideration in this connection is of importance. 



Out of the 22 species of minerals classified as zeolites by 

 Dana in his System of Mineralogy there are but 11 which, on 

 account of their abundance or chemical composition, need con- 

 sideration here. The theoretical composition of these, as indi- 

 cated from a comparison of several to many analyses, is shown 

 in the accompanying table. In addition to the true zeolites are 

 included several other hydrous silicates closely related, both as 

 regards chemical composition and mode of occurrence, and which 

 in the present discussion cannot well be excluded. 



