COLL. GEOLOGY. 63 



contortions produced in the beds, where their course 

 is irregular or transverse. The smaller granite veins, 

 whether parallel or transverse, have most generally a 

 fine grained structure with the aspect of an ordinary 

 granite, although this rule is far from absolute. 

 But both the regular and the irregular gneiss are tra- 

 versed by larger veins which produce an occa- 

 sional disturbance in each. In the irregular they only 

 add a little to the general confusion, and even in the 

 regular beds they seldom produce so much displace- 

 ment as materially to divert them from their general 

 tendency, unless where they are very numerous. These 

 veins have almost always a peculiar aspect, containing 

 a large proportion of felspar with but little quartz, the 

 mica being also either sparingly dispersed or altogether 

 absent. The concretions of felspar are often of great 

 size, the substance itself having a high glassy lustre : the 

 veins also frequently assume the graphic character. 

 Towards the southern end of Coll veins of similar dimen- 

 sions occur among the gneiss, disposed in the same man- 

 ner, but formed of a peculiar rock, which I slightly men- 

 tioned in describing Tirey. The basis of this substance is 

 a quartz possessing considerable transparency and an 

 oily lustre, and containing crystals of red felspar thinly 

 disseminated. Similar compound occurs in the gneiss 

 of Sky. 



There remain still to be described two circumstances 

 which are important towards the general history of 

 gneiss. Distinct beds of mica slate are found regularly 

 alternating with it in several places. A still more re- 

 markable appearance is that of a bed similarly situated 

 and consisting of a conglomerated rock formed of frag- 

 ments of quartz imbedded in a micaceous schist ; offering 

 an example of a breccia in a situation where these have 

 not been supposed to exist. This rock is to be seen in a 

 small bay near Ben Feoul. 



It is unnecessary, after the remarks on that subject in 



