Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/147

 by the intrusion of some other rock. The uniformity of their dip seems a sufficient reason to reject the former explanation, and their present appearance is more probably derived from both the last mentioned causes acting on different points. Concerning the action of water or other similar causes we can only conjecture, but of the latter we have occasional proof in the actual existence of masses of trap rock overwhelming them in some places, and doubtless concealing them entirely in others. It may perhaps be owing to this rock and to its subsequent destruction only, that their present state is to be referred. I shall therefore conclude these illustrative remarks by a few words respecting the trap rocks which occur in the vicinity of Cruachan, and which are in many places so intimately connected with these detached portions of the secondary strata.

The nearest mass of these substances is a long mountain ridge which occupies part of the northern shore of Loch Etive, descending towards the western sea and skirting the plain of Connel. It is in this place well known to all who have travelled the west Highlands, since the road passes under large rocks of the conglomerate which is connected with it. It is equally familiar to those who visit Oban, since the surrounding country and the neighbouring islands are covered with more or less extensive masses of it, in some cases reposing on the older schists, which form the visible basis of this country, in others upon the sandstone strata already described. It is in this latter case that it produces the effect already alluded to of partially concealing the masses of sandstone, so as to give an appearance of separation where no real one exists. In some cases it is also probable that it has actually dislocated and separated them, intruding among them as all the rocks of this family do, from below. Hence it assists us in explaining the state of the sandstone formerly described,