Mac CuUoch on Piichstone. 47 



found in the CuchuUin hills, the outer laminae of which are 

 pitch-stone, graduating into the common maiter of the vein- 

 But Lamlash presents the most striking instance of this nature : 

 here a vein of basalt passes through a vein of greenstone ; and 

 the outer parts or walls, to the depth of about half an inch, are 

 formed of a glossy black pitch-stone. In some parts of its course 

 this outer lamina is very decided and distinct, but in others it gra- 

 duates into the basalt of the vein by an intermediate substance 

 similar to that which forms the Scuir of Egg. A confirmation 

 of this peculiarity, though of a less marked nature, is found in 

 one of the well-known veins of Egg, where the outer part of 

 the vein is a remarkably perfect and brittle pitch-stone, while 

 the inner is formed of the intermediate substance just de- 

 scribed. 



From all these circumstances it may be concluded that pitch- 

 stone possesses a strong affinity to the trap rocks, both in its 

 geological relations and in its origin, being among the latest 

 of the intruding rocks. 



In one respect it presents an important character of difference. 

 While in all cases veins of trap are very persistent, and can in 

 many places be traced to a fundamental mass, those of pitch- 

 stone are remarkably limited in extent, and have never yet 

 been found connected with any principal body of the rock. 



Veins of pitch-stone vary considerably in breadth, ranging 

 from many yards to a few inches. In the case above alluded 

 to, where basalt veins become pitch-stone during the progress 

 of ramification, they descend almost to the dimensions of a 

 thread. 



In composition they are not always and invariably the same 

 throughout. Independently of the rare cases in which veins 

 composed of basalt in the middle are formed of pitch-stone at 

 the sides, it sometimes happens that pitch-stone of many differ- 

 ent qualities will be found in the same vein. In these cases 

 the most perfect or vitreous-like parts are placed as in the 

 former instances, at the outer sides of the vein ; and, in the 

 same cases, they present differences of colour as well as of 

 texture and aspect. 



