220 



ECONOMIC GEOLOGY 



the green sulphate of iron. During the process the sulphuric 

 acid has attacked the alumina in the shales. These solutions 

 traversing limestones would find the calcium carbonate reacting 

 upon the aluminum sulphate solutions according to the equation 



The gelatinous precipitate thus formed is carried upward in 

 spring basins where it finally settles as a pisolitic mineral. In 



FIG. 116. Map showing the occurrence of corundum as a differentiation 

 product of peridotite. (After Thomas and MacAlister's Geology of Ore 

 Deposits.) 



the absence of the calcium carbonate to react upon the hydrous 

 sulphate of aluminum the alums would be formed in the above 

 process of alteration. Waters containing the sulphates of iron 

 and aluminum form deposits of these salts in close proximity to 

 their points of origin. They are commonly found as stalactites 

 and even as incrustations. Alunite and alunogen are the com- 

 monest species thus generated. They are found around mineral 

 springs and in the shafts or tunnels of mines. 



The silicates of aluminum occur both as primary and as 



