492 A TREATISE ON METAMOKPHISM. 



The source of the most abundant vapor emitted during volcanic action 

 has been much discussed. Some have held that the water is derived 

 from the original magmas, never having been at the surface, while others 

 have held that it is derived from the water of the surface and underground 

 circulation. It may be in part derived from each. Chlorine, hydrochloric 

 acid, hydrofluoric acid, and hydrosulphuric acid are undoubtedly largely 

 formed by the action ot hot water upon chlorides, fluorides, and sulphides. 

 Sulphurous and sulphuric oxides are produced by the action of the oxygen 

 upon the sulphides. These may unite with water and produce sulphurous 

 and sulphuric acid. To what extent the above chlorine, fluorine, and sulphur 

 compounds are derived directly from the magmas and to what extent from 

 later segregations containing an unusual amount of these elements it is not 

 my purpose here to discuss. The hydrogen of fumaroles and volcanoes is 

 usually regarded as clue to the decomposition of water." This may be 

 accomplished by ferrous oxide according to the following equation: 



H 2 + 2FeO=H. 2 +Fe 2 3 . 



The immediate source of a part of the hydrogen is probably hydrosul- 

 phuric acid, the hydrogen being liberated as this acid decomposes and the 

 sulphur separates. But since the H 2 S is probably produced, as already 

 explained, by the action of H 2 on sulphides the hydrogen is indirectly 

 derived from the water. It is shown that carbon dioxide is extensively 

 liberated by the process of silication in the zone of anamorphism. (See pp. 

 g77_679.) It is natural to suspect that this liberated carbon dioxide largely 

 makes its way to the surface at places where there is active upward circu- 

 lation. Volcanic districts are certainly such places. Hence it is believed 

 that much of the carbon dioxide issuing in connection with fumaroles and 

 solfataras is that liberated by silication. Where carbonates are present as 

 rocks adjacent to volcanoes another portion of the carbon dioxide is doubt- 

 less derived by the action of the strong acids upon these carbonates, salts 

 of the strong acids being produced and the carbon dioxide liberated. No 

 opinion is expressed as to the relative abundance of these two sources of 

 carbon dioxide. 



As would be expected, the ordinary reactions of the belt of weathering 

 take place in regions of fumarolic and solfataric action the same as else- 



«Geikie, Sir Archibald, Text-book of Geology, Macmillan & Co., London, 1893, 3d edition, pp. 

 193-197. 



