526 PITCHSTONE. 



doubt that it is a vein from which the including 

 parts have been removed by the ordinary causes 

 of waste. In one respect, however, the veins of 

 pitchstone differ materially from those of trap; 

 as they have never yet been found connected with 

 large masses, in this country at least ; being, on 

 the contrary, of very limited extent, and, at pre- 

 sent, independent. 



These veins are found situated in granite, in 

 sandstone of recent origin, and in the trap rocks 

 which lie above it ; a variety of connection which 

 affords sufficient evidence of their intruding cha- 

 racter, and which is also sufficient to prove, 

 d priori^ that no pitchstone is stratified ; as there 

 is no instance in nature, of a stratified rock 

 forming veins, or occurring in connections so 

 distant and discordant as these. 



The strong analogy subsisting between pitch- 

 stone and basalt, is shown by other circumstances. 

 In two instances, at least, one of which occurs in 

 Lamlash, and the other in Sky, veins of basalt 

 are found, at their outer walls, to pass into this 

 substance by a gradation more or less perfect ; 

 and, in the veins found in Egg, an imperfect 



