1112 A TREATISE ON METAMORPHISM. 



shall take place a very large amount of ferrous salt is necessary. Similar 

 reactions may be written with the ferrous reducing salt in other forms than 



sulphate. 



The class of reactions represented by the above equations has not been 

 confirmed by experimental work in the laboratory. It was suggested by 

 geological facts. In the basic volcanic rocks which have, been profoundly 

 altered by metasomatic changes in the belt of cementation secondary sul- 

 phides and magnetite are very generally present in close association. Such 

 occurrences are very well illustrated by the ancient volcanics of the Lake 

 Superior region — as, for instance, the Hemlock volcanic formation of the 

 Crystal Falls district of Michigan." For these rocks it can hardly be as- 

 sumed that organic material or reducing agents other than the ferrous salts 

 are present. But ferrous silicates, and, therefore, ferrous compounds, are 

 verv abundant; hence the suggestion that they are the agents which have 

 resulted in the reprecipitation as sulphide of the sulphate compounds which 

 have come down from the belt of weathering. The widely disseminated 

 association of pyrite and magnetite in these rocks suggests that even the 

 following reaction may possibly take place: 



22FeSO 4 +20H 2 O=FeS,+7Fe 3 O 4 +20H,SO, 



If heat be liberated by this change, there is good chemical reason for 

 its occurrence. But it must be admitted that this reaction is highly specu- 

 lative and needs confirmation by experimental work. 



At those places where the sulphates enter trunk channels, mingling of 

 the solutions may cause precipitation when one of the solutions bears reduc- 

 ing agents. It is believed, however, that the precipitation of sulphates 

 as sulphides in the trunk channels of circulation is far more frequently 

 accomplished by hydrogen sulphide. 



An important source of hydrogen sulphide is the reaction of organic 

 matter upon the sulphides. Of such sulphides, that of iron is, of course, of 

 the greatest consequence. Confirming the conclusion that the reaction of 

 organic matter upon the sulphides may produce lrydrogen sulphide is the 

 fact that artesian waters held in sediments containing organic material 

 are frequently marked by the presence of hydrosulphuric acid. This is 



« Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan: 

 Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 73-154. 



