402 ARRAN. GEOLOGY. OVERLYING ROCKS. 



imperfections of the map of Arran, prevents any accu- 

 rate references. The predominant varieties ^appear to 

 be indurated claystone and clinkstone, which, either in 

 a simple state or slightly porphyritic, occupy by far 

 the larger portion of the ground, displaying sometimes 

 the prismatic and at others the schistose structure ; while 

 they are occasionally of a dark blue colour, but generally 

 of a pale greyish white. In proceeding to point out 

 such places as admit of a more accurate description, it 

 will be perceived that these occur chiefly along the shore, 

 where considerable sections are exposed to view. 



A remarkable porphyry was mentioned as forming the 

 promontory of Drumodune. This is a large bed, ex- 

 tending, as its broken front, about 500 yards, and 

 being there, on an average, about eighty feet in thick- 

 ness. It is divided into prisms perpendicular to the 

 bed, which give it, in a general view, a columnar 

 aspect; but they are neither defined nor regular. It 

 lies upon the sandstone strata, which can be seen 

 in a horizontal position in different places ; but is sepa- 

 rated from them by a laminar intermediate mass which 

 is not porphyritic. This consists of the basis of the 

 porphyry without the felspar crystals ; approaching to 

 compact felspar, and of different colours, iron grey, brown, 

 and pale grey ; all of which are occasionally found in 

 the porphyry itself. To illustrate the nature of this rock, 

 it is here necessary to mention, that to the westward of 

 this point a large vein of the same porphyry occurs on 

 the shore, the sand of which prevents its connexion with 

 the surrounding rocks from being traced. But on the 

 east side of Drumodune, towards Tormore, at a place 

 conspicuous for pitchstone veins, another of a very large 

 size, and of the same nature, is to be seen traversing the 

 sandstone ; together with some smaller veins which it 

 is unnecessary to specify more particularly. The most 

 extensive mass of this porphyry lies between Black- 

 water bay and Sliddery, where its broken edge can 

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