48 i Dr. Mac Culloch on Quorlz Rock, 



parallel mica, so thin as not to be discovered in the cross fracture. 

 From Fassafern, Balahulish and Tyndrum. 



34. The same rock, but the layers of mica more conspicuous arid 

 undulating. From Balahulish. 



35. The same passing into mere mica slate. From the series at 

 Balahulish and Tyndrum. These demonstrate the alternation of 

 the quartz rock with mica slate. 



36. A large grained breccia consisting of fragments of quartz 

 cemented by a mass of earthy white felspar, approaching to por- 

 celain clay. From Jura. 



37. A rock consisting of felspar and granular quartz, with here 

 and there an atom of blue slaty clay interspersed among the grains. 

 From Jura. 



38. Various specimens of the same, with the clay slate increasing. 



39. The same approaching still nearer to gray wacke, with a basis 

 of clay slate, and ultimately terminating in it. These two from 

 Jura. 



These specimens show the transition into common gray wacke slate. 



40. The same composition of rock, but in which mica slate holds 

 the place of clay slate. 



Different specimens of this variety demonstrate the passage of 

 quartz rock into that sort of graywacke which I have described 

 under the head of Aberfoyle. From Jura. 



41. The same rocks passing by insensible degrees into a conglo- 

 merate, which will by some be ranked with common graywacke. 

 From the same place. 



From a comparison of the several characters under which this 

 rock appears in the same place, from the similarity of these 

 characters or of a majority of them in the different situations in 

 which it occurs, there need be no hesitation in concluding that 



