OVERLYING ROCKS. 495 



both in veins and in mountain masses. It is 

 frequent in the former shape, and is even some- 

 times thus found accompanying' the apparently 

 antient porphyries that traverse granite. 



The colours are similar to those of the pre- 

 ceding variety. Specimens, of which the colours 

 are brilliant, sometimes pass by the name of 

 jasper, to which they approximate. 



E. Clinkstone. This is still harder than the last 

 variety, and is not easily scratched by the knife. The 

 fracture is also more perfectly splintery and conchoidal ; 

 occasionally somewhat granular, while the lustre is more 

 considerable, and the fragments are slightly translu- 

 cent on the edges. It is considerably sonorous when 

 struck, but not so as to be distinguished by this cir- 

 cumstance from many other rocks of the trap family. 

 Its mineral definition was formerly given. 



a. Massive. 



b. Columnar, or prismatic. 



c. Laminar. 



The dark varieties of b have also, like the 

 former, been enumerated among the basalts. 

 The same remarks as in the two preceding cases, 

 may be made on var. c. 



It is also found in similar geological situations, 

 and the colours are equally various. It fre- 

 quently acquires an arenaceous aspect on the sur- 



