SILICEOUS SCHIST. 561 



although carbon is intimately combined with it 

 in those cases where it has originated in bitumi- 

 nous shale. 



Organic remains sometimes occur in the se- 

 condary varieties, giving rise to the unfounded 

 opinion, already noticed, that these exisetd in ba- 

 salt, for which this rock has occasionally been 

 mistaken. They are rare, and commonly so far 

 altered in their forms, that their zoological cha- 

 racters cannot be assigned. 



The colours of both varieties are, most com- 

 monly, tones of grey, varying from an impure 

 white to black ; and they are often distinguished, 

 in the same specimen, by a striped disposition, 

 the consequence of laminar alternation, 



SYNOPSIS OF SILICEOUS SCHIST. 



FIRST DIVISION. 

 Primary. 



A. Simple in appearance, and resembling clay 

 slate, but distinguishable by its extreme hardness. 



o o 



