132 Dr. Mac Culloch's observations 



clay slate, and limestone, cease at the place where Glenco first begins 

 to contract its dimensions as we proceed from Balahulish eastward. 

 From this point till we arrive at the King's house, or near it, the 

 mountains on each side consist of porphyry, or of the different 

 simple substances which form its bases, these simple rocks being 

 indeed much more prevalent ' than those which are, strictly speak- 

 ing, porphyritic. As these hills subside at their eastern end in the 

 moor of Rannoch, the mass of porphyry disappears, and is suc- 

 ceeded by the granite already described ; but veins of all the dif- 

 ferent varieties are found connected with it, every where intersect- 

 ing the latter rock, and although gradually diminishing in frequency 

 as we recede from the great mass, still admitting of being traced even 

 to its very extremity. I have no means of assigning the boundary of 

 this porphyry towards the north, but as the same rock is found to 

 form the summit of Ben Nevis, it is probable there is some connec- 

 tion, more or less interrupted, between them. It is easily seen that 

 towards the south it forms both the mountains which go by the 

 name of Buachaille Etive, the conoidal and acute forms of which 

 are visible from a great distance throughout the surrounding country. 

 From these it extends along the eastern side of Corrich y bae, but as 

 the southern sides of their declivities have not been examined, it is 

 impossible at present to state the limits here between the porphyry 

 and the granite which I have conjectured in a former part of this 

 paper to extend from those mountains to Cruachan. 



The vertical structure of this rock explains the cause of the abrupt 

 and perpendicular faces which give the peculiar character to the dark, 

 solemn, romantic Glenco. Together with that it presents the same 

 laminar tendency which is so remarkable in the rock of Devar, de- 

 paper.* In many places there is an appearance 



* Geol. Trans. Vol. 2. 



