Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/142

 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 speaking, porphyritic. As these hills subside at their eastern end in the moor of Rannoch, the mass of porphyry disappears, and is succeeded by the granite already described; but veins of all the different varieties are found connected with it, every where intersecting 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 connection, 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, described in a former paper. In many places there is an appearance