46 TJliEY. GEOLOGY. 



whole country ; but the conclusion respecting its univer- 

 sality is not deduced from a general view only, portions 

 having been examined from one extremity of the island 

 to the other. It is possible that some beds of the 

 other rocks which occur in gneiss may have escaped my 

 observation, but this must ever happen, as no length of 

 time would suffice for the fracture and examination of 

 every rock which protrudes over a surface of this nature 

 and extent. Under the gray crust of lichen, which wraps 

 all in one undistinguishable covering, mineralogical 

 treasures may yet be concealed in Tirey as in many other 

 places. 



The composition of this gneiss is various, but the 

 varieties are all included in that division of it which also 

 forms lona. Generally speaking, it is characterized by 

 the presence of hornblende. Mica is 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. The 

 prevailing composition therefore is hornblende, quartz, and 

 felspar. In some places it is very perfectly foliated, in 

 others it approaches so near to granite, that its nature 

 can only be discovered by a favourable fracture. The 

 quartz occasionally disappears, in which case the rock 

 sometimes puts on an appearance intermediate between 

 gneiss and granite; which gradually passes, by varying 

 into a finer grain, or by the exclusion of more of the 

 felspar, into a simple hornblende schist, or into that 

 mixture of hornblende and felspar which has been called 

 primitive greenstone ; in some cases to a mere unfoliated 

 hornblende rock. It is also not uncommon to find in it 

 larger or smaller laminae, or beds and lumps of horn- 

 blende rock. There is no regular progress from the granitic 

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

 mixed, without order or arrangement. The varieties 

 in which hornblende predominates appear however 

 more abundant at the southern end of the island, 



2 



