4& TIREY. GEOLOGY. 



pected of having been derived from veins. When the 

 felspar is large it has most usually a glassy aspect ; 

 occasionally it is of a brick-red colour, as are also many 

 of the granitic varieties of the gneiss. These redder 

 varieties are most abundant on the west side near Bal- 

 phetrish, and in the vicinity of Cornag more. 



In the gneiss there occurs a singular rock, consisting of 

 quartz with imbedded crystals of felspar : but as it is more 

 common in Coll, I shall delay the description of it for the 

 present. The granite of the veins contains in general 

 the four ingredients, quartz, felspar, mica, and hornblende. 

 Green compact felspar, although much rarer than in lona, 

 is nevertheless found in it. In the vicinity of Trava bay I 

 also observed the same ingredient entering the gneiss, thus 

 forming a beautiful rock consisting of alternate laminae of this 

 substance, and of common felspar mixed with hornblende. 

 In one or two places there occurred among the veins 

 specimens worthy of notice for the colours of the felspar 

 and of the quartz which they contained. In one of 

 these the felspar was of a pale indigo blue and of a 

 glassy lustre. In the other the quartz was of a sea green 

 and considerably translucent. In both cases these sub- 

 stances were in small concretions. 



The gneiss of Tirey is more remarkable for contain- 

 ing masses of limestone. One of these has long been 

 known by the flesh-coloured marble which it affords ; of 

 which a quantity has been exported for the purpose of 

 ornamental architecture since the time when it was first 

 pointed out by Raspe. It is improperly called a bed, as 

 it is only an irregular rock, lying among the gneiss 

 without stratification or continuity. In this respect 

 it resembles the greater number of the primary lime- 

 stones found in gneiss and in mica slate, and may be 

 considered as a large nodule. There is considerable 

 obscurity attending these detached masses of limestone. 

 Of all the rocks which occur in extended masses, granite, 



