268 PITCHSTONE. 



vein. If this occurrence is as yet solitary, it must be 

 recollected that it does not happen, perhaps even in 

 one trap vein of a thousand ; so that it may be merely 

 a question of chances, depending on the much greater 

 rarity of veins of pitchstone. The fact, even if soli- 

 tary, is valuable ; as establishing, in another impor- 

 tant particular, the resemblance between pitchstone 

 and the trap rocks. 



There is no stronger proof of the common origin of 

 pitchstone and trap, than the transition from that sub- 

 stance to basalt, which is sufficiently common. The 

 Scuir of Egg holds a place intermediate between the 

 two. Nor are similar transitions rare in basaltic veins ; 

 particularly where one traverses another of a larger 

 size. In Sky and elsewhere, the basaltic veins which 

 ramify through the hypersthene rock, become gra- 

 dually finer in texture as they divide, until the minuter 

 filaments are converted into perfect pitchstone. In 

 the same place, there is a large basaltic vein gradua- 

 ting into pitchstone at its side; and in Lamlash, where 

 a vein of basalt passes through one of greenstone, the 

 outer parts, to the depth of half an inch, are formed of 

 a glossy black pitchstone. In some places, this outer 

 lamina is very decided and distinct; but, in others, it 

 graduates into the basalt of the vein, by an interme- 

 diate substance, resembling that of the Scuir of Egg. 

 And in one of the veins in this island, the outer part 

 is a perfect and brittle pitchstone, while the inner is 

 formed of the same intermediate substance. With 

 this identity of geological relations and origin, there is 

 this important difference, that while veins of trap are 

 very persistent, and are always connected with some 

 fundamental body, those of pitchstone are limited in 

 extent, and have not yet been traced to a principal 

 mass. 



