COLL. GEOLOGY. 6l 



THERE are some differences between the geological fea- 

 tures of Coll and Tirey, although gneiss constitutes the 

 mass of this island also from one extremity to the other. 

 No limestone, an object always of great research for 

 agricultural purposes, has yet been observed, although 

 it is not improbable that among the thousands of grey 

 rocks which cover its surface, some calcareous beds or 

 masses may yet be latent under the covering of grass, 

 heath, and lichens. 



The gneiss is disposed in rocky hills, sometimes crowd- 

 ed, at others distinct, of an elevation seldom or never 

 attaining 500 feet, interspersed with smaller protuberances, 

 and extending over the whole surface of the island. 

 Towards the southern and western parts it exhibits in 

 many places a regular disposition in beds, of which the 

 alignment can often be traced for a considerable space, 

 straight, and free from all that disturbance which marks 

 the rocks of Tirey. Towards the middle and the nor- 

 thern end of the island this order is not so visible, the 

 masses appearing to be disposed in an irregular and dislo- 

 cated manner. The direction of the beds, where it can be 

 traced, is north-easterly, and their dip is toward the east. 

 Their elevation is generally considerable, often reaching 

 to seventy or even to eighty degrees, and seldom de- 

 clining so low as to fifteen. 



The mineral composition of the regular and irregular 

 beds is not essentially distinct, though their external 

 aspect differs : hornblende is found in all, as in Tirey. 

 The laminar structure is sometimes evanescent ; in which 

 cases the rock borders so nearly on granite as to be 

 scarcely distinguishable from it. In other places it passes 

 at one extreme into hornblende schist or into an 

 ordinary hornblende rock, while at the other, by 

 the exclusion of the quartz and hornblende, it becomes 

 merely a mass of felspar. Between these two there are 

 seen numerous and nameless varieties, differing both in 

 texture and composition. But it is most usual to find 



