850 A TREATISE ON METAMORPHISM. 



thus the quartz particles be so rearranged that their maximum and mean 

 diameters are in a common plane, producing quartz-schist. 



It has already been explained (pp. 818-819) that where silica is 

 rearranged it is sure to be segregated, because any area of chert attracts 

 materials of like kind to itself, and large masses grow at the expense of small 

 ones. Even in the cases where the cherts are undoubted replacements the 

 source of the silica may largely have been sponge spicules, diatoms, etc., 

 which were disseminated through the limestones. This material can not be 

 discriminated from that furnished by the ground waters coming from some 

 other formation. The essential fact is that such a deposit is a replacement 

 deposit, whether the silica be derived from the decomposition of silicate, by 

 the solution of sponge spicules, or in some other maimer. 



In order that the silica may be concentrated, it is of course necessary 

 that the material previously occupying the place shall be dissolved. Such 

 concentration and solution are especially likely to occur in those formations 

 which originally contained a considerable amount of cherty material, and 

 which were readily soluble. The best representatives of rocks of this class 

 are the carbonates — calcareous, magnesian, and ferruginous. At the time 

 the chert is segregated the carbonate is dissolved. It therefore follows that 

 in the calcium-magnesium carbonate family and the iron-bearing carbonate 

 family the most extensive deposits of chert should be expected; and with 

 this the facts correspond. 



Observation shows that many carbonate formations have been exten- 

 sively silicified. Positive evidence of the replacement of carbonates by 

 chert is furnished by formations which contain numerous silicified fossils 

 which were originally calcareous, such as the mollusks and corals. Where 

 fossils of this kind, now chert, are contained in a mass of chert, it is natural 

 to suppose that the formation was in the main originally calcareous. A 

 good illustration of extensive replacement of calcareous formation by silica 

 is that of the Tampa Bay chalcedony pseudomorphous after coral. 



Where replacement has been extensive, the average compositions of 

 considerable masses of rocks have been greatly changed — indeed, where 

 pure limestone is replaced by chert, entirely changed. There is no means 

 by which one can estimate the amount of silica that has been introduced 

 ' from an outside source in the silicificatioii of great carbonate formations. 

 Doubtless the amount of such material is great, but probably. in most 



