560 SILICEOUS SCHIST. 



in Seal pa, in the schists which alternate with the 

 primary sandstone ; and it is more particularly 

 described in that work to which it has been so 

 often found necessary to refer in justification of 

 many of the views here held out. 



The texture, and, consequently, the fracture 

 of siliceous schist, is very various. In the pri- 

 mary, it is often similar to that of the analogous 

 primary strata, particularly where it passes into 

 hornblende schist ; a transition which sometimes 

 occurs, although on a very limited scale. In the 

 secondary, the texture is always very compact, 

 although the aspect of the fracture .varies from 

 that of common flint to the more dull appearance 

 of compact limestone. The form of the fracture, 

 in the primary varieties, presents no peculiarities ; 

 but, in the secondary, it is flat, or minutely angular 

 and flat, or conchoidal, or perfectly conchoidal, 

 like that of flint, with an equal degree of lustre. 



In the primary varieties, quartz, or mica, may 

 be contained as constituent minerals, just as they 

 are in the unaltered schist, when that has been of 

 a compound or mixed nature ; but, in the secon- 

 dary, no visible mineral has yet been found, 



