IONA. .GEOLOGY. 15 



be found to present many circumstances interesting to the 

 science of geology. It is difficult to reduce their descrip- 

 tion to geographical order, from the want of names and 

 of a detailed map ; but I shall adopt what appears the 

 most intelligible method, and trace them by the line of 

 the coast. 



From the Town to beyond the Bay of Martyrs, the 

 prevalent, or perhaps the sole rock, is a very black 

 compact clay slate, which occasionally contains horn- 

 blende, and, in some few instances, mica. There is, 

 however, no real micaceous schist, or at least I did not 

 observe it. It has been imagined to exist here, because 

 most of the tombs and crosses are carved in a stone of 

 this kind. It appears rather, that like the sand-stone in 

 the groinings of the arches and the ornamental carvings 

 of the soffits and capitals, it was brought from the main 

 land, where it abounds along a considerable range of the 

 western coast. 



Not far below the town and a little before the clay 

 slate terminates, a vein of black basalt is found tra- 

 versing it; the only instance of this kind which fell in 

 my way throughout the island. 



I must now remark, that the opposite coast of Mull 

 consists of a red large-grained granite, which has been 

 much used in building the Cathedral. A few rocks of 

 this substance are found on the neighbouring shore of 

 lona, below the Bay of Martyrs ; one of which approaches 

 so near to it as to be in contact with the schist: they 

 are very evidently continuations of the rock of Mull. 

 The schist at this place puts on a remarkable appearance. 

 Externally, it displays a singular mixture of black and 

 red. On breaking, it is found to be possessed of unusual 

 hardness, the fracture being as acute and cutting as 

 that of siliceous schist. In many places it loses its black 

 colour and becomes grey ; in others, it is partially mixed 

 and mottled with red felspar ; which at length increasing 

 in quantity, it appears on the very verge of passing into 



