220 A TREATISE ON METAMORPHISM. 



In a third class of changes quartz may be wholly replaced by other 

 minerals, as by magnetite and hematite. Very frequently the deposition of 

 the new minerals seems to be conditioned upon the solution of the quartz. 

 The replacement of quartz by iron oxide is illustrated in the Lake Superior 

 region in both the iron-bearing and the slate formations." 



The most favorable conditions for the solution of silica, especially of that 

 formed by the decomposition of the silicates by carbonation, are furnished 

 by the belt of weathering. The most favorable conditions for the deposi- 

 tion of silica as quartz are those of the belt of cementation. The solution of 

 silica in the belt of weathering of the zone of katamorphism and its deposi- 

 tion in the belt of cementation of this zone is perhaps the best illustration 

 of the principle explained on pages 634-636, that material dissolved in 

 the belt of weathering may be extensively deposited in the belt of 

 cementation. Recrystallization of quartz mainly takes place in the zone 

 of anamorphism, although it undoubtedly occurs to some extent in the 

 zone of katamorphism, and especially in the belt of cementation. The 

 process of silication takes place almost invariably with decrease in volume, 

 provided all the compounds concerned are solids. Where the carbonates 

 are silicated the decrease in volume ranges from 20 to 40 per cent. Silica- 

 tion occurs upon a great scale in the zone of anamorphism — is, indeed, one 

 of the most distinctive chemical reactions of that zone. 



TEIDTMITE. 



Tridymite: 

 Si0 2 



Hexagonal, or pseudo-hexagonal. 

 Sp. gr. 2.28-2.33. 



occurrence. — Trirjymite usually occurs as an autogenic mineral in cavities 

 in lavas, such as rhyolite, andesite, trachyte, etc. 



Modifications. — Tridymite is dissolved more readily than quartz. The 

 material of tridymite may go through any of the changes which silica of 

 quartz may pass through, with the difference that its recrystallization would 

 result in the production of quartz (rhombohedral ; sp. gr. 2.652-2.654) 

 rather than the original mineral, tridymite. The changes of tridymite into 

 other minerals than quartz need not be discussed in detail, since the reac- 

 tions are the same as with quartz, except that the volume decrea.se is greater 



"Van Hise, C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan: Mon. U. S. 

 Geo!. Survey, vol. 28, 1897, pp. 370, 400-405. 



