172 A TREATISE ON METAMORPHISM. 



of iron. Iron occurs extensively in the ferrous form, in magnetite, in car- 

 bonates, and in silicates. To a considerable extent it occurs as a sulphide. 

 To a small extent it occurs as metallic iron. In all of these forms it is 

 capable of oxidation. The main result of the oxidation of these com- 

 pounds, so far as the iron is concerned, is to change the monoxide to 

 ferric oxide. But where it is present as a sulphide it may be changed to a 

 sulphate, and then be thrown down as a basic ferric sulphate. Ferric oxide, 

 hydrous or anhydrous, is an important constituent in the sedimentary 

 rocks, and its presence is, without doubt, largely due to oxidation in 

 the zone of katamorphism. To a far less extent other metals, such as 

 copper, lead, zinc, etc., occur in the native form, in partially oxidized 

 forms, or as sulphides. All these substances may be oxidized. These 

 substances have little importance in general geology, but are of great 

 importance in the production of ores. All of the reactions of oxidation 

 take place with great liberation of heat and with increase of volume. In 

 the zone of anamorphism partial or complete deoxidation of the highly 

 oxidized compounds may occur. The ferric iron mar be reduced to the 

 ferrous form. The sulphates of iron and the other metals may be reduced 

 to sulphides. In most cases the reducing agent is organic matter. The 

 reduction of the metals by organic compounds results in the oxidation of 

 the carbon and hydrogen, thus producing carbon dioxide and water. The 

 carbon dioxide and water largely escape. Where reducing agents are not 

 present the highly oxidized materials produced in the zone of katamorphism 

 commonly remain in this condition even if the material passes into the 

 zone of anamorphism. Deoxidation can not, therefore, be. said to be char- 

 acteristic of the zone of anamorphism to the degree that oxidation is 

 characteristic of the zone of katamorphism. The reducing reactions all 

 take place with great absorption of heat, so far as the metals are concerned, 

 and with decrease of volume. However, since heat is liberated by the 

 oxidation of the carbon and hydrogen, it is probable that the sum total 

 of the heat reaction in deoxidation in the zone of anamorphism is to 

 liberate heat. 



In the matter of oxidation and deoxidation, the zone of katamorphism 

 presents a case in which the chemical law of the liberation of heat controls, 

 without reference to change in volume, while in the zone of anamorphism 

 the pressure tending to produce decrease of volume and chemical reactions 

 with the liberation of heat probably work toa*ether. 



