240 A TREATISE ON METAMORPHISM. 



all gradations, but, as noted below, an approach to the former extreme 

 probably is the common case. 



Recrystallization of calcite and dolomitization take place on the most 

 extensive scale at all depths and under both mass-static and mass-dynamic 

 conditions; they are therefore alterations which are common to both 

 physical-chemical zones. By dolomitization it is believed that great masses 

 of calcite have been transformed to dolomite. The evidence of this trans- 

 formation and the detailed facts in connection with the change are given 

 under dolomite. (See pp. 798-808.) The fact that dolomite forms in both 

 zones would be sufficient evidence that the reactions producing this com- 

 pound liberate heat, even if this had not been experimentally determined 

 to be the fact. It has been pointed out before (pp. 181-182) that the 

 formation of dolomite is a typical illustration of an alteration in which both 

 the volume and the chemical changes liberate heat, and which therefore 

 may occur in all zones and belts of the lithosphere. 



The change from calcite to wollastonite occurs chiefly or wholly in 

 the very deep-seated rocks, especially in the zone of anamorphism. In this 

 zone, as noted (pp. 764-766), it can not be assumed that material is added 

 in considerable quantity from an outside source by circulating water; hence 

 in this zone silica for the change is believed to have been a solid. The 

 reaction is therefore one taking place with the absorption of heat and 

 condensation of volume. The silication of calcite to wollastonite in the 

 zone of anamorphism may be taken as a typical example of the heat and 

 volume change of silication of carbonates in that zone. 



occurrence. — The chief source of dolomite is believed to be the dolomiti- 

 zation of calcite (see pp. 238-239), but dolomite is also a direct chemical 

 precipitate. Dolomite also forms in subordinate amount by the alteration 

 of ankerite. The ultimate source of the magnesium carbonate for the 

 dolomitization of the calcite is the magnesium liberated by the carbona- 

 tion of the silicates in the zone of katamorphism, especially in the belt 

 of weathering. The reactions for the decomposition of some of the 

 simple silicates, such as diopside and tremolite, are given under those 

 minerals. The magnesium for the dolomitization need not be directly 

 derived from a silicate, but may be from the solutions of the sea or from 



