470 BUTE. GEOLOGY. 



loidal, occur in the same vein ; but in all the cases 

 described, these different substances were disposed in a 

 laminar direction, or parallel to the sides of the vein. 

 But, above Kilchattan bay, an instance occurs, where 

 several varieties are to be seen, disposed, although in a 

 somewhat irregular manner, across the vein, which is here 

 about six feet wide. The substances thus found are, 

 greenstone porphyry, containing very large crystals of 

 glassy felspar, a tufaceous rock, a very fine basalt decom- 

 posing into balls, and common greenstone. 



The large vein, before mentioned as appearing to pro- 

 ceed from the mass of trap at Ascog, presents a singular 

 and, at first sight, a very deceptive appearance.* It is 

 laminar, or divided into layers parallel to its sides, and is 

 at the same time vertical. At the inner edge, or west 

 side, (its course being north) it appears to contain frag- 

 ments of limestone occupying one of the laminae, accom- 

 panied by some larger parallel portions of the same 

 substance. A close examination discovers the true nature 

 of this appearance without diminishing its singularity. 

 It is thus found that the outer laminae, which appear, like 

 the inner, to be parts of the vein, are not trap, but shale ; 

 and that the limestone fragments are contained in this 

 substance, not in the trap. This shale is so fine and 

 compact that it can scarcely be distinguished from a 

 basalt; but pursuing it further outwards from that part of 

 the rock which, from its greater hardness and permanence, 

 seems to appertain to the vein, it is found to be succeeded 

 by a coarser shale alternating with sandstone, still vertical, 

 or parallel to the real laminae of the vein. These differ in 

 no respect from the ordinary shale and sandstone that form 

 the shore ; but which, every where else, are in their usual 

 horizontal position. Not far off, the same shale, contain- 

 ing fragments of limestone, are found together with the 

 simple shale and the sandstone, in that which may be 



* Plate XXI. f.g. S. 



