CONTACT METAMORPHISM. 493 



where. Indeed, these reactions probably take place at a greatly acceler- 

 ated rate. Therefore in areas of fumarolic and solfataric action oxidation, 

 carbonation, hydration, and solution are important reactions, but ordinarily 

 this is not recognized because of the more striking action of the exceptional 

 compounds which are present in unusual abundance. That oxidation 

 takes place on an extensive scale is shown by the yellow, red, or brown 

 colors of the altered rocks due to limonite, hematite, and magnetite which 

 extensively form from the monoxide of iron. Hydration in connection with 

 fumaroles and solfataras has been to a considerable extent overlooked. 

 Delesse found that dry steam at a pressure of five atmospheres had no 

 appreciable action upon minerals. Barus found that the diabase of the 

 Comstock lode, containing fresh feldspar, was not kaolinized by the long- 

 continued action of steam at 100° C. a However, it is rare that steam is 

 dry in the actual case of fumaroles and solfataras, and therefore, as would 

 be expected at high temperatures, the process of hydration goes on at a 

 great rate. Carbonation also takes place on a scale scarcely less extensive 

 than hydration. In consequence the formation of sodium carbonates is 

 very common and in some places it is found in large quantities. 



However, it is not by these common products but by the exceptional 

 products that fumarolic and solfataric action is especially distinguished. 

 The chlorine and hydrochloric acids form chlorides, of which sodium chlo- 

 ride (NaCl), potassium chloride (KC1), ammonium chloride (NH 4 C1), iron 

 chloride (FeCl 3 ), copper chloride (CuCl 2 ), manganese chloride (MnCl 2 ), 

 and other chlorides have been observed. Through the action of sulphuric 

 acid various alums are formed, of which potassium-aluminum sulphate 

 and sodium-aluminum sulphate are the more common. By action upon 

 calcium-bearing compounds sulphuric acid forms gypsum. By action 

 upon the alkali-bearing compounds the sulphuric acid forms Glauber's 

 salt, or sodium sulphate, and potassium sulphate. The action of hydro- 

 sulphuric acid upon the various compounds may form various sulphides, 

 or it may be decomposed without action upon other substances, in which 

 case it liberates hydrogen and deposits sulphur. The hydrofluoric acid 

 attacks the various compounds, forming various fluorides, of which fluorite 

 is the best known. The boric acid forms borates. Of these various sub- 

 s' Barus, C, On the thermal effect of the action of aqueous vapor on feldspathic rocks: School of 

 Mines Quart., vol. 6, 1884, pp. 1-23. 



