PITCHSTONE. 527 



transition of a similar nature may also be seen. 

 In the same island, the remarkable vein called the 

 Scuir, already mentioned, holds a place so inter- 

 mediate in texture between basalt and pitchstone, 

 as to be but imperfectly referable to either. The 

 same circumstance, on a smaller scale, is visible 

 in many other places ; and it is further remarka- 

 ble, that, in Sky, where veins of basalt sometimes 

 ramify into slender filaments, these gradually 

 become pitchstone. 



The last instance of this analogy, is shown 

 by the fact, yet solitary, it is true, which I have 

 elsewhere described, that, in one of the veins in 

 Arran, fragments of the including sandstone 

 strata are imbedded in the vein. 



Veins of pitchstone vary much in breadth, as 

 they do in their other dimensions ; attaining one 

 of many yards, and, in some cases, not exceeding 

 the size of a thread. In the same vein, the mine- 

 ral characters also vary ; so that many of the 

 varieties hereafter enumerated, are sometimes 

 found associated, either by transition, or in a 

 laminar manner, in the same vein. Transitions 

 into .chert or chalcedony, are also found under 



