MINERALOGY; WJTII A CLASSIFICATION OF SILICATKS. 



67 



cupric oxyd, lime, magnesia and even baryta. The ratio of silica to the bases varies 

 widely in species having the same proportions of protoxyd and sesquioxyd bases ; and, 

 moreover, species having otherwise the same chemical composition, and similar external 

 characters, differ in the presence or the absence of water. Finally, the degree of conden- 

 sation, as shown liy the value of Y, varies considerably among phylloids, all of which are 

 nevertheless so well marked in their physical characteristics that the micaceous or phylloid 

 t)'pe is one of the first recognized by the student. 



X Tribe 9. Pbotoperphylloid. 



§ 88. We have spoken of micas and dilorites, but the distinction is an arbitrary one, and 

 the transition from hydrous magnesian micas like some j)hlogopites, to chlorites, is not so 

 great as that from these same micas to those of the muscovitic type, often themselves 

 hydrated. The difficulties of adequately discussing this great tribe are increased by the 

 fact that different formulas are assigned to minerals having the same specific name. Phlog- 



