454 Dr. Mac Cullocu on Quartz Rock, 



6. Grey quartzy rock, with a basis of a compact splintery ap- 

 pearance, containing imbedded grains of transparent quartz. 



7. Small grained breccia, of which the basis is an earthy-looking 

 mixture of felspar and quartz, resembling claystone, or compact 

 felspar, and uniting angular and rounded fragments of quartz. 



8. An uniform aggregate of large grains of felspar and of trans- 

 parent quartz, without the aspect of granite, since all the grains 

 appear to have been mechanically rounded and re-united. 



This quartz rock in the several varieties now described, although 

 as I have already remarked, it forms the essential and fundamental 

 part of the island, does not occupy it to the exclusion of all others. 

 Beds of a rock resembling both mica slate and graywacke, of com- 

 mon graywacke, of finer graywacke slate, and of perfectly fine 

 and uniform clay slate, together with beds of chlorite slate, appear 

 in various places, all difficult to trace through their whole bearings, 

 yet all apparently superimposed on the quartz rock. It is necessary 

 to describe these rocks somewhat more particularly, as they are inti- 

 mately connected with the quartz rock, and serve to illustrate its 

 history. The following are the most remarkable varieties : 



1. A mixture of quartz in grains, with mica slate, of a character 

 intermediate between quartz rock and mica slate, or rather resem- 

 bling some of the varieties of that graywacke which I have described 

 in my account of Aberfoyle, (p. 447.) 



2. The same, of a much larger grain, with distinct scraps of 

 mica slate. 



3. The same, with the mica slate so predominant that the com- 

 pound forms a dark rock, in which the grains of felspar and quartz 

 bear a small proportion to the slate. 



4. The mica slate still increasing, and the texture still granular. 



5. The same, with a slaty fracture. 



