THE GANGUE MINERALS. 1129 



ered, the deposits of a first concentration by ascending waters. It lias been 

 explained in various parts of this treatise that the two great rock-making 

 acids are carbonic acid and silicic acid, and further it has just been explained 

 that in the deep-seated zone carbonic acid is extensively produced. Thus 

 it is entirely natural that so far as the metals are somewhat soluble and can 

 travel as carbonates, they will do so. This applies especially to the alkalies 

 and the alkaline earths, and to iron and manganese. When carbonate solu- 

 tions containing these various metals rise in the trunk channels of circula- 

 tion with diminishing temperature and pressure, it naturally follows that the 

 less soluble of the carbonates are thrown down in large amounts. These 

 less soluble carbonates are calcite, dolomite, siderite, and rhodochrosite. 

 The carbonates of sodium and potassium are not thrown down, because 

 these compounds are so readily soluble. 



In conclusion it may be said that the carbonates, as original deposits 

 by ascending waters, are essentially gangue minerals, not ores. So far as 

 I know none of them are mined as ores within the United States, although 

 in some places siderite and rhodochrosite, as deposits of the first concentra- 

 tion by ascending waters, may be an unimportant source of iron and man- 

 ganese ores. 



Excellent illustrations of rhodochrosite as a gangue mineral are fur- 

 nished by the San Juan district, Colorado. This carbonate is generally 

 found in that area, but is an especially important and abundant gangue 

 mineral at the Smuggler Union mine. 



SILICATES. 



The silicates are well known to be important gangue minerals deposited 

 by the deep circulation. If one were to give a list of the silicates which 

 have been found as gangue in veins in various parts of the world, it would 

 comprise a large proportion of the entire list of silicates. This is an inevi- 

 table consequence of the facts that, first, the dominant acid of nature is 

 silicic acid, and second, the silicates for the most part are rather insoluble, 

 many of them extremely so. It has been explained at various places that 

 silica as colloidal silicic acid travels in great quantities through the rocks, 

 being produced on an enormous scale in the belt of weathering- by the 

 process of carbonation. (See pp. 173-177, 480.) Silicic acid formed 

 in this manner and released by other reactions thus enters the belt 

 of cementation and finds its way to the trunk channels of circulation, to 



