468 BUTE. GEOLOGY. 



This mass contains in one place a vein, presenting ail 

 appearance, which however common it may be in mineral 

 veins, is not so in this rock. This vein is a complicated 

 calcareous one, the greater part of it consisting of a breccia 

 formed of fragments of the trap cemented by the calca- 

 reous matter. It is probably the effect of subsidence 

 and fracture, by which detached pieces have fallen into a 

 cavity and been subsequently enveloped by infiltration. 



The principal mass thus described, is superincumbent, 

 near the shore, on a softer and clayey substance ; which is 

 also stratified and parallel to it, with a common inclination. 

 This is succeeded by a bed of coal, which will be described 

 hereafter, and that by a second bed of the same clayey 

 rock, below which there is no further access to any alter- 

 nations that may exist. This rock is of a pale lead grey, 

 and of a hardness intermediate between that of the earthy 

 amygdaloids and dry clay. It decomposes by a kind of 

 exfoliation, into balls and irregular masses ; and is often 

 porphyritic, the felspar being as tender as the base ; as if 

 the whole had undergone together some change from the 

 original hardness of a trap porphyry. No similar sub- 

 stance has occurred among the numerous trap rocks 

 examined in the course of this survey. The dip of these 

 becis is westerly at fifteen degrees, and they are not visible 

 in the other parts of the hill ; nor indeed would they have 

 been exposed here but for the workings made in the coal. 



There is a trap vein near the mass now described, 

 lying in a north and south direction, and nearly vertical. It 

 is probably a branch from that ; but this cannot be proved, 

 since the shape of the land, and the nature of the shore, 

 prevent the connexion from being traced. It may indeed 

 be a vein of a later date than the overlying trap, this 

 occurrence being common ; in which case there would be 

 two distinct formations of trap in Bute, as in Sky and 

 other places ; but I could not trace any instance of a vein 

 actually traversing the trap of the Garroch head ; which, 

 being a sufficiently extensive mass, might be expected to 



