Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/424

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Dr. Mac Culloch on the Geology of

elucidate still further the disposition of Schihallien, of Jura, and of the north of Scotland. I shall merely indulge in a few very cursory remarks on this part of the formation of Isla.

I have stated in another place that in Jura the quartz rock assumes towards the shores a disposition far less regular than that which it exhibits in the elevated districts. On the sides of the narrow strait which separates this island from Isla, this irregularity is particularly remarkable, and it is also very easy of access. These rocks possess all the varieties of structure which I have formerly described, and with them mica slate and clay slate are found in beds, which, even if this question had not already been decided by evidence drawn from other places, I should have no hesitation in calling alternate.

The shores of Isla which form the other side of this narrow strait, resemble those of Jura so perfectly, and correspond with them so nicely in their various characters, that a spectator would not, without a geographical mark for his guide, be able to determine which of the two he was examining. We may, without hazarding too strong a conjecture, look back to a time when these islands formed one continuous tract. Veins of basalt and granite are to be observed in both, the former by their superior permanence bearing a resemblance to high walls, which at a distance emulate ancient ruins; appearances similar to that which forms the supposed volcanic amphitheatre of Mull, so highly pictured by Faujas de St. Fond. Some varieties of this basalt exhibit by an incipient decomposition, a structure consisting of thin lamellæ, and occurring, although in a less remarkable degree, at the Giant's Causeway. There is an example of this structure among the specimens which are the subject of the present brief commentary. But to proceed to the great limestone district of Isla. Mr. Necker's