68 A DESCRIPTION OF THE [Ch. V. 



are also so much like those of granite, as at first sight to 

 mislead an observer : but a careful examination will always 

 detect, somewhere, an indication of a foliated structure. This 

 is particularly the case about Kilbride; but the gneiss re- 

 sumes the more common foliated character, as it approaches 

 the shores of Loch Boisdale. In lona and Tirey, the gneiss 

 generally contains hornblende, the mica being more rare: 

 and although it is found even together with the hornblende, 

 it is most generally observed in the vicinity of the granite 

 veins by which the gneiss is traversed. In some places, the 

 gneiss is perfectly foliated ; but in others, it approaches so 

 near to granite, that its nature can only be discovered by a 

 favourable fracture. There is no regular progress from the 

 granitic to the schistose beds, but they seem to be every where 

 mixed without order. Indeed, the gneiss generally, in Tirey, 

 does not exhibit any regularity of disposition, or even such 

 an approach to it, as to show the probable course of the beds : 

 or if, for a small space, any line of direction be traced, at the 

 very next step the appearance of regularity vanishes.* 



The beds of gneiss are generally disposed in the direction 

 of N.E. and S.W., in conformity with the other primary 

 stratified rocks in this part of Scotland, dipping towards the 

 east at various angles. But it often happens, that the direc- 

 tion of these beds is very irregular, and accompanied by 

 every possible variation of dip. Thus, in Rona there is on 

 the eastern side a great sequence of straight beds, dipping 

 uniformly towards the east ; while on the western, they lie in 

 a contrary direction ; the two meeting in the middle of the 

 island, like the roof of a house. In other places, the same beds 

 will be found reversed ; the eastern beds dipping to the west, 

 and vice versa : or they undergo changes still more intricate, 

 displaying wavings and contortions. These irregularities, 

 however, are more frequently limited to the granitic gneiss.f 



Since mica is an ingredient in gneiss, it is not surprising 



* Western Islands of Scotland, vol. j. pp. 45, 46. f Idem, vol. i. p. 213. 



