414 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, with- 

 out 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 lamellae, and occurring, although in 

 2L 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 



