494 A TREATISE ON METAMORPHISM. 



stances formed, some are volatile and may be sublimed. Among these 

 ammonium sulphate, sulphur, and boric acid are well known. 



While any of the minerals present may be acted upon, on the average 

 the silicates are decomposed on a far greater scale than other compounds ; for 

 silicates are the dominant salts present which the active agents find to work 

 upon. Besides producing the various compounds above mentioned, which 

 are especially characteristic of fumarolic and solfataric action, the 

 relatively insoluble residual products of the silicates are extensively 

 produced, such as kaolin, etc. While these chemical reactions are taking 

 place, concurrently with and depending upou them the rocks are softening, 

 and the result is to produce a whitish or yellowish earth, frequently stained 

 red where iron oxide is abundant. So long as the fumarolic and solfataric 

 action continues, mingled with this residual material are many of the soluble 

 compounds mentioned above; but after fumarolic and solfataric processes 

 cease, the soluble salts are rapidly leached out by the ground water circu- 

 lation and there remains the residual, relatively insoluble compounds char- 

 acteristic of the belt of weathering. 



In summary it may be said that fumarolic and solfataric action is not 

 so different from the ordinary reactions of the belt of weathering as might 

 be supposed. The reactions are essentially the same. All of the reactions 

 characteristic of the belt of weathering occur, but at a more rapid rate 

 than ordinarily. Moreover, the local concentration of numerous strong 

 acids makes their action of relatively greater importance than under the 

 ordinary conditions of the belt of weathering. Finally, in. the case of fuma- 

 roles and solfataras the process of decomposition is wholly chemical, 

 whereas ordinarily in the belt of weathering organic compounds play an 

 important part. 



In a strict sense fumarolic and solfataric action, so far as it is the 

 action of gases, is that of mineralizers, as defined by Naumann. However, 

 fumarolic and solfataric action as here considered is not that which is ordi- 

 narily known as the action of mineralizers ; hence I have avoided the term. 



RELATIONS OF DISINTEGRATION TO DECOMPOSITION AND SOLUTION. 



The mechanical work of weathering is the physical subdivision of the 

 material; in other words, is disintegration. The chemical work of weather- 

 ing is decomposition and solution. The process of disintegration promotes 

 the chemical work of decomposition and solution, because, as disintegration 



