242 A TREATISE ON METAMORPHISM. 



salt for that of the calcium and magnesium. The oxide of iron is an illus- 

 tration of a pure replacement, not of an alteration. 



The formation of diopside, tremolite, and wollastonite is known to 

 occur in deep-seated rocks, and especially in connection with mass- 

 mechanical action where the rocks are deformed by fiowage. As 

 repeatedly noted, in the zone of anamorphism the circulation of water is 

 reduced to a minimum; and it can not be supposed that important addi- 

 tions are made from the outside, and therefore the silica must be supposed 

 to have been previously present in the rocks. Indeed, we know that silica 

 usually accompanies deposits of calcite and dolomite; hence I conclude that 

 the reactions take place with substantially the decrease in volume above 

 assigned to the changes. In the reactions heat is absorbed. The changes 

 are therefore again typical illustrations of silication in the lower physical- 

 chemical zone. 



ANKERITE AND PARANKERITE. 



occurrence. — All the compounds from normal ankerite and parankerite to 

 the extremes of composition given above (p. 237) are included under the 

 general term ferro-dolomite, which I have elsewhere used as covering all 

 the ferriferous compounds standing between dolomite on the one side and 

 siderite on the other. (See p. 823.) 



The sources of ankerite and parankerite are the same as siderite, 

 with the difference that at the time of the formation of the iron carbonate, 

 calcium and magnesium carbonate are present, or formed, and unite with it. 



Alterations. — The more common alterations of ankerite and parankerite 

 are to limonite (amorphous; sp. gr. 3.80), hematite (rhombohedral ; sp. 

 gr. 5.225), and magnetite (isometric; sp. gr. 5.174), the calcium and 

 magnesium carbonates either separating or simultaneously undergoing the 

 alterations given under "Calcite" and "Dolomite." Equations may easily 

 be written for any definite compound by which the iron carbonate passes 

 into the minerals mentioned in the same way that siderite does and the 

 calcium-magnesium carbonates separate. The volume changes are in the 

 same direction, and the physical conditions under which ankerite and par- 

 ankerite alter to limonite, hematite, and magnetite are the same as those 

 for the alteration of siderite to the like compounds. Therefore the equa- 

 tions and summary of physical conditions will not be here repeated. 



Other important alterations of the ferro-dolomites are to sahlite (mono- 



