1128 A TREATISE ON METAMORPHISM. 



temperatures and pressures which exist in the deeper parts of fissure veins, 

 this reagent would result in anhydrous stannic oxide, or cassiterite; but 

 this is a mere suggestion. The conditions under which anhydrous 

 stannic oxide is precipitated from its solutions should be experimentally 

 investigated. 



In conclusion it may be said that one oxide which very probably is 

 deposited by ascending' waters as an ore is cassiterite. 



CARBONATES. 



Aside from hydrosulphuric acid, the acid of great importance in the 

 deposition of ores is carbonic acid. This, as is well known, is indeed 

 the dominant acid contained in issuing underground waters. This point 

 both Le Conte" and Posepny 6 strongly emphasize. I have already pointed 

 out sources for the excess of carbon dioxide held in the underground 

 waters. (See p. 678.) 



A very interesting confirmation of the theory given on the pages 

 referred to regarding the liberation of silica by the process of carbonation 

 near the surface and the liberation of carbon dioxide probably by the 

 process of silication at depth is furnished by the Geyser mine of Custer 

 County, Colo., described by Emmons. c Here waters were analyzed from 

 the 154-meter and the 615-meter levels. The surficial waters contain ten 

 times as much silica as the deep-seated waters, and the deep waters hold a 

 greater quantity of carbonic acid than the vadose circulation. 



The carbonates which are most extensively deposited in the veins of. 

 the first concentration are those of calcium and magnesium as calcite 

 (CaC0 3 ) and dolomite (CaC0 3 . MgC0 3 ). While calcite and dolomite are 

 the dominant carbonates in the gangue, the carbonates of the other alka- 

 line earths, strontianite (SrC0 3 ) and witherite (BaC0 3 ), are also well known. 

 The other important carbonates are those of iron — siderite (FeC0 3 ) — and 

 of manganese — rhodochrosite (MnC0 3 ). Of course, it is well known that 

 carbonate of zinc (smithsonite) and carbonates of copper (azurite and mala- 

 chite) occur abundantly as ores; but these are mainly secondary products 

 of descending waters and do not come under the subject now being consid- 



«Le Conte, Joseph, Genesis of metalliferous veins: Am. Jour. Sci., 3d ser., vol. 26, 1883, p. 11. 

 ftPosepny, F., The genesis of ore deposits: Trans. Am. Inst. Min. Eng., vol. 23, 1894, p. 237. 

 o Emmons, S. F., The mines of Custer County, Colo.: Seventeenth Ann. Kept. U. S. Geol. Survey, 

 pt. 2, 1896, pp. 460-464. 



