SEIL. GEOLOGY. 131 



schistose rocks and of the trap which form their most con- 

 spicuous features. 



It is not necessary to describe the trap in great detail, 

 after the account already given of Kerrera ; to the corre- 

 sponding rocks of which it not only bears a striking 

 resemblance, but of which it may be said to form a de- 

 tached portion. As far as can be discovered, it is every 

 where incumbent on the conglomerate and schist, as in 

 that island; the junction being also often attended by 

 ramifying veins. It breaks into bare precipices with an 

 irregularly prismatic fracture, and is thence easily distin- 

 guished at a distance, from the rocks that constitute the 

 remainder of the island. The predominant substance 

 appears, as in Kerrera, to be a compact felspar, generally 

 very brittle though hard, and breaking with great violence 

 into sharp fragments. It has sometimes a laminar ten- 

 dency ; a circumstance attending the rocks of this tribe, 

 which will again be noticed more particularly when Arran 

 shall come under review. Occasionally also it contains 

 a crystal of felspar or a minute nodule of zeolite ; thus 

 marking the first approaches to the porphyritic and to 

 the amygdaloidal characters. It varies much in colour, 

 being grey, brown, brownish purple, and dark lead blue ; 

 and in one case the brown varieties were found reticulated, 

 after exposure to the air, with extremely minute white 

 lines, although these were invisible in the fresh fracture. 

 Veins of the same material are abundant in the island, 

 and they present the same ambiguity of disposition as 

 those of Kerrera. It is sufficient to say that they occupy 

 every possible direction, either conforming to the strata 

 or traversing them. But the conclusions to be drawn 

 from these appearances have been already generally stated 

 under the account of Kerrera, and will be more usefully 

 detailed in describing Torsa, where they occur with greater 

 distinctness. I shall here notice a few only that present 

 remarkable features. 



One of these is a vein of well characterized black basalt, 



