562 SILICEOUS SCHIST. 



B. Containing mica, and distinguishable, in the 

 same way, from the micaceous schists, or micaceo-ar- 

 gillaceous schists (graywacke), which it otherwise re- 

 sembles. 



C. Containing quartz sand, but similarly charac- 

 terized by its extreme induration. 



These varieties present but trifling distinc- 

 tions of colour, resembling the several schists 

 from which they are derived. As they vary in 

 the degree of induration, so they also pass gra- 

 dually into the ordinary micaceous and argilla- 

 ceous schists. 



SECOND DIVISION. 

 Secondary. 



A. With an earthy dull fracture and the aspect 

 of shale ; distinguishable only by its hardness. 



B. The fracture somewhat glossy; more indu- 

 rated. 



These two varieties retain the laminar struc- 

 ture of shale. 



C. More highly indurated, more brittle and glossy ; 

 the laminar structure disappearing, and the fracture 

 splintery and small conchoidal. 



The three preceding varieties are either black, 

 or of different shades of grey. The different 



