416 MR JOHN S. FLETT ON 



flagstone, and hence cannot be the same as that already described to overlie the thick 

 agglomerate bed, a few yards further to the east. 



Among the yellow sandstones, about two miles further to the west along the shore, 

 and about a hundred feet below where they pass into the red sandstones, occurs another 

 belt of contemporaneous volcanic rock. It is associated here, also, with a series of flag- 

 stones, and no doubt is on the same level as the rocks just described. In a little bay to 

 the east of the Castle, a bed of dark green ashy sandstones, mostly fine-grained, but with 

 here and there lapilli of a couple of inches in diameter, is to be seen, interbedded with 

 yellow sandstones and flags. It is very similar in character to the ash beds in 

 Shapinshay which overlie the lava ; but while these are mostly of very inconsiderable 

 thickness, it is in some places three or four feet thick. No lava is associated with it, 

 and in bhe sandstones above and below I found no trace of any recurrence of the volcanic 

 activity. In all probability it is the representative, in this section, of the coarse 

 agglomerate already described, which must have greatly thinned out in the intervening 

 distance. The striking feature of this volcanic zone is its very diminutive thickness. 

 Still, the occurrence in Orkney of such a zone is a remarkable confirmation of the opinion 

 expressed by Sir A. Geikie, that the " ancient volcano of John o' Groats might be one of 

 a series which might hopefully be sought for among the Orkney Islands." * 



Rocks of an intrusive origin occur also in this district, the principal mass being 

 exposed in the locality last mentioned, about 50 yards west of the ashy sandstone. It 

 forms a mass of about 25 feet in thickness, though its base is not exposed, a dark green 

 rock, which is first seen in the shore, and runs out to sea in a series of picturesque stacks 

 and reefs. Its intrusive character is shown by the absence of any amygdaloidal upper 

 surface, and the evidently unconformable junction with the overlying sandstones. Yet 

 these were, so far as I. could make out, not markedly altered, though they are so decom- 

 posed that this would not be easy to determine. The rock is about 30 feet beneath the 

 ashy sandstone, and in structure is a much weathered diabase, with crystals of plagioclase 

 felspar, augite, and probably olivine, almost entirely decomposed into green chloritic 

 products, which show traces of ophitic structure. Throughout Deerness, in several 

 places, occur masses of volcanic rock so decomposed and so obscured in their geological 

 relations by the surface accumulations that it is not easy to form an opinion as to their 

 true character. They all occur among the yellow sandstones and the flags associated 

 with them. One is seen to the south of the Free Church, and several outcrops are 

 known in the vicinity of the Public School. I am greatly indebted to Mr Magnus 

 Spence for specimens and observations on these outcrops. From their microscopic 

 structure and the absence of any accompanying tuffs, they are in all probability intrusive 

 sheets. The freshest specimen I obtained was a dark green diabase, with well-marked 

 ophitic structure and pseudomorphs of serpentine after olivine. It came from a deep 

 pit, at one time sunk in a field behind the Public School.t 



* Sir A. Geikie, " Old Red Sandstone," Trans. Roy. Soc. Edin., vol. xxviii. p. 406. 



t The Black Holm of Copinshay consists of an intrusive sheet of olivine diahase about 30 feet thick, enclosing a large 

 hi of baked Hag penetrated by numerous veins. This is probably that referred to by Jameson, Scottish Islands, ii. p. 235. 



