PITCHSTONE. 531 



structure, pitchstone sometimes gives indications 

 of an internal arrangement, which, as in the case 

 of the trap rocks, is only detected on weathering. 

 Many of the varieties become of a pure white on 

 the surface after exposure to the air ; scaling off 

 in successive crusts that resemble white enamel, 

 as they still retain the vitreous aspect and fracture. 

 At the same time, the exposed surfaces are, co- 

 vered with undulating lines, resembling certain 

 varieties of marbled paper, and evidently result- 

 ing from some corresponding difference of lami- 

 nar structure ; the lamina most sensible to at- 

 mospheric action, being thus detected, although 

 invisible in the fresh rock. In a further progress 

 to decomposition, pitchstone is at length con- 

 verted into a fine clay, forming, in water, a very 

 tenacious paste ; and it appears to be very readily 

 acted on by the water, as may be witnessed in 



Arran. 



/ 



It does not always weather to a white enamel, 

 becoming occasionally brown, and, more rarely, 

 turning into a black powder. Occasionally, it 

 exhibits the kind of bloom seen on a plum, or on 

 certain kinds of bottle glass. 



i>I M 2 



