158 MR CUNNINGHAM ON THE GEOGNOSY OF 



" several pieces of black pitchstone, much resembling 

 that found in the islands of Arran and Mull." Dr MacCul- 

 loch, in his Description of the Western Islands, remarks, 

 that " on the beach near Eilan Chastel, rolled stones of a 

 black pitchstone-porphyry," differing in aspect from the 

 other pitchstones of the island, present themselves. On 

 examining this part of Eigg, we noticed numerous boul- 

 ders of the same rock, and at last were so fortunate as to 

 discover, in situ, the mass from which they had been de- 

 rived. This pitchstone cannot be examined throughout 

 its whole extent ; but it appears to be one of the ends of 

 a great vein traversing the basalt ; the removal of the ab- 

 sent portions having been effected by the united actions 

 of the waves and atmosphere. In one place, a portion 

 of the basalt seems to be entangled in the pitchstone. 

 The external characters of this rock are as follows : — 

 Colour, very dark bluish-black ; lustre, resinous ; struc- 

 ture, imperfectly slaty and slightly porphyritic, from con- 

 taining crystals of glassy felspar ; fracture, slaty in the 

 large, and small conchoidal in the small ; transparency, 

 none.* This vein is represented on Plate V. fig. 2. 



the recent species of acer." The vessels are stated to be numerous, 

 and much compressed, and to have the elliptical or circular form of 

 most dicotyledonous woods. The cellular structure cannot be discern- 

 ed, and all the sections which this observer made, presented, where the 

 vascular structure is visible, well-defined annual layers. Mr Nicol con- 

 cludes by remarking, that the specimens of wood from Mull were the 

 only examples of dicotyledonous plants which he had seen from rocks 

 of the secondary class. (The rocks from which the wood of Mull is 

 derived belong to the oolitic group.) 



* Viewing pitchstone as a member of the trap and porphyry series, 

 it must be considered as a rock of rare occurrence, when compared 

 with the other members of these great natural families, and in Scot- 

 land, if we except the instance of the Scuir, one which is never met 

 with in masses of a size sufficient to constitute mountains, but is in- 

 variably found as veins. As this rock is one of interest in a geo- 

 gnostical point of view, and desired by the mere collector of specimens, 



