61 Dr. M'^CuLLOcus Additional Remarks on Glen Tilt. 



contaminated with mica, like the beds formerly described by the river side, 

 and might probably, if fully oj)cned, prove valuable, as it exists in consider- 

 able (juantity, and in an apparently undisturbed state^ the granite being far 

 distant. 



In a former paper, published in the second volume of the Geological 

 Transactions*, 1 have described a bed of limestone containing hornblende, 

 which occurs at Loch Laggan. As this circumstance is not common^ I may 

 point out another example of it, to be observed within the tract at present 

 under consideration : it is in one of the beds which cross from the Tilt 

 through the grounds of Lude. The rock is of a pale dove colour, and of a 

 large grain; and is intermixed throughout with very minute particles of black 

 hornblende, not exceeding in size a pin's point. 



Quartz Rock. — I have on difierent occasions shown that quartz rock often 

 contains associated beds of breccia, in which respect it possesses an analogy 

 to the secondary sandstones. In these cases the breccia consists of fragments 

 of the same original rocks, which, in a more comminuted state, form the finer 

 parts of the deposit, and is evidently of the same period of formation. I had at 

 that time found no breccia connected with quartz rock itself, and composed of 

 fragments of that rock reunited ; but such a one occurs near the Fender, in 

 the hilly ground south of the Tilt. Such local breccias are not uncommon 

 among the secondary rocks, and they are particularly well known among 

 the calcareous strata, where they form a considerable class of ornamental 

 mabrles. 



In the case of quartz rock, as well as in those now mentioned, the breccias 

 must be supposed to have originated in some violent motion or fracture of the 

 beds, without any material change of place ; the parts being subsequently 

 reunited by siliceous infiltrations, in a manner already pointed out in an article 

 on quartz rock published in the fourth volume of the Geological Transactions f. 



Gneiss. — In my first communication on Glen Tilt I questioned the occur- 

 rence of this rock as an extensive member of the strata which lie on the granite. 

 There appears no reason to change this statement, as far as relates to the 

 hills which bound the southern side of the valley, and which alone were 

 described in the paper. But the case is otherwise with respect to the hills on 

 the northern side, which I had not then examined ; since gneiss occurs there 

 in considerable abundance, together with quartz rock, micaceous schist, 



* I'agf 435. t Pag^ 266. 



