CHEMICAL WORK IN ZONE OF ANAMORPHISM. 677 



SILICATION. 



Iii Chapter IV (p. 168) silication has been defined as the union of silica 

 with bases so as to produce silicates. In that chapter the close analogy 

 between the silicates and the carbonates has been pointed out, as well 

 as one of the most fundamental facts of metamorphism, their mutual 

 interchange. (See pp. 173-177). In the chapters on the zone of katamor- 

 phism it has been seen that the carbonation of the silicates is one of the 

 processes of fundamental importance, and in the belt of weathering the one 

 which has the most far-reaching effects. It has further been explained 

 that in the zone of anamorphism this process is reversed, silica replacing 

 carbon dioxide of the carbonates and producing silicates. While silica in 

 the zone of anamorphism unites with bases not previously combined with 

 carbon dioxide, it is probable that silication of this kind is unimportant. It 

 is certain that much of the free silica which unites with bases in the zone 

 of anamorphism simultaneously drives off carbon dioxide. There are a 

 number of silicates which are formed by the direct silication of a single 

 mineral. To illustrate: silication of calcite forms wollastonite; silication of 

 dolomite forms tremolite and wollastonite ; silication of ankerite forms sahlite 

 and actinolite; silication of siderite forms grunerite. Frequently silication 

 requires two or more minerals to produce the new silicate. As instances, 

 we have the silication of rutile and calcite together, producing titanite; of 

 hypersthene and calcite, producing' actinolite and anthojmyllite ; of olivine 

 and calcite, producing actinolite ; and of dolomite and siderite, or ferrous 

 dolomite, producing anthophyllite. Silication usually does not occur alone, 

 but takes place in connection with deoxidation or dehydration, or both. As 

 a case where we have silication with dehydration may be mentioned the 

 silication of gibbsite, producing sillimanite and cyanite. A case of silication 

 with deoxidation and dehydration is the formation of griinerite from limonite. 

 But usually the instances of silication combined with deoxidation or dehy- 

 dration, or both, are so complex that it is impossible to state what particular 

 combinations of minerals are deoxidized, dehydrated, or both, and silicated 

 in order to produce a definite silicate. 



It will be seen in Chapter IX, when rocks are considered, that this 

 process of silication takes place on a vast scale. Indeed, no sooner does a 

 carbonate pass from the zone of katamorphism to the zone of anamorphism 



