496 OVERLYING ROCKS. 



face after exposure to the weather ; so as even to 

 have been confounded with sandstone by incau- 

 tious observers. 



As these substances C, D, E, pass into each 

 other by insensible gradations, it is not always 

 possible decidedly to refer a specimen to either. 

 This is irremediable, but it does not destroy the 

 utility of the preceding distinctions. 



F. Compact felspar ; including the hornstone of 

 some mineralogists. This is distinguished from the 

 last substance, to which it is nearly allied, by its supe- 

 rior hardness, compactness, and lustre. The edges of 

 the fragments are also more decidedly translucent. A 

 fuller description was formerly given. 



a. Imperfectly laminar. 



b. Massive, with a smooth, splintery, and 



conchoidal fracture. 



c. Crystalline-granular. 



This substance var. a, b, occurs occasionally 

 in veins in a simple state, but is more frequently 

 slightly porphyritio in some part of its course. I 

 know not that it is found in mountain masses or 

 beds among the trap rocks of secondary origin. 



Var. c appears under many different aspects, 

 in consequence of the varying fineness of its 

 texture. It occurs among the secondary traps, 



