I] 



ROCK-FORMING MINERALS 



21 



alumina, and are rarely to be detected in the soils 

 resting upon sedimentary rocks. 



N Hornblende and Augite, though differing in crystalline 

 shape, are chemically identical, and consist of silicates of 

 varying proportions of lime, magnesia, alumina, ferrous 

 and ferric oxides ; manganese and the alkali metals are 

 generally also present They constitute, with plagioclase 

 felspar and magnetic oxide of iron, the chief part of the 

 rocks that are sometimes roughly termed " greenstone " — 

 basalts, diorites, etc., of both volcanic and plutonic 

 origin. They decompose under the action of carbonic 

 acid charged water, especially those containing much 

 lime, while those with much magnesia are the most 

 resistent; the products of the action are kaolinite, 

 oxides of iron, and carbonates of lime and magnesia. 

 The following analysis (Ebelmar) show the chemical 

 change in the weathered layers of a basalt from 

 Bohemia and a greenstone or dolerite from Cornwall : — 



The loss amounts to about 44 per cent in the 

 case of the basalt, and 34 per cent in that of the 

 greenstone. 



Another example may be given of the analysis 



