SERPENTINES OF SCOTLAND. 313 



south end of Holyhead Island. Here dark-green serpentine is 

 associated with gabbro. The serpentine is shattered and slickensided, 

 and is in contact with ophi-calcite, which is a breccia of dark serpen- 

 tine cemented by calcite infiltrated from the overlying Carboniferous 

 limestone. Under the microscope the serpentines give evidence of 

 being altered olivine rocks like Dunite, with various other associated 

 minerals now decomposed and replaced by ill-preserved pseudomorphs, 

 so that it is classed by Professor Eonney as an altered peridotite. 

 The rock at Porthdinlleyn in Caernarvonshire, formerly regarded as 

 serpentine, is classed by Professor Bonney as a diabase tuff on the 

 evidence of its microscopic structure. 1 



Serpentine of Ayrshire. On the coast of Ayrshire serpentines 

 are exhibited at Lendalfoot and at other points between Balcreuchan 

 Port and Pinbane Hill. They are also seen at Balhamie Hill, near 

 Colmonell. In the latter exposure the rock has a rhomboidal joint- 

 ing, the joints coated with greenish or whitish steatite. It is full of 

 crystals of glittering bronzite or some similar mineral. The olivine is 

 completely converted into serpentine, and the rock is regarded by 

 Professor Bonney as an altered olivine enstatlte which contains 38 

 per cent, of silica, 35 per cent, of magnesia, and 4 per cent, of 

 alumina. The associated dolerite rocks are particularly interesting, 

 some of them having the aspect of being metamorphosed sediments, 

 while others have an igneous aspect. Some writers have regarded 

 them as diorites, but they contain plagioclase and augite. There are 

 also gabbros of two ages, one similar to the gabbro of the Lizard, very 

 rich in diallage, so as to be almost a mass of diallage crystals. The 

 serpentine is older than the diallage rock and gabbro, and is intruded 

 into rocks which were regarded by Murchison as of Bala age. 2 



Serpentine dykes of Carboniferous age occur in Forfarshire. 



These serpentines, like those of Portsoy in Banffshire, 3 are modi- 

 fied volcanic rocks, though really to be classed as metamorphic rocks. 

 Other serpentines appear to be derived from sedimentary rocks, meta- 

 morphosed in the usual way. (See Heddle, T.E.S., Edin., vol. xxviii.) 



Secondary Volcanic Hocks. 



Sir Henry De la Beche, nearly fifty years ago, gave an excellent 

 account of the felsitic rocks associated with the lower part of the New 

 Red Sandstone in Devonshire. He states that the section at Wash- 

 field near Tiverton gave the best evidence of a volcanic eruption, the 

 lava being covered by detritus containing angular volcanic fragments 

 which sometimes weigh a ton. These fragments contained quartz 

 and large crystals of glassy felspar ; 4 and nearly thirty years later Mr. 

 "W. Yicary, F.G.S., described all the volcanic rocks near Exeter in 

 more detail. 5 There are no more interesting volcanic rocks in Britain. 



1 Bonney : "Serpentine of Anglesey," Q. J. G. S., vol. xxxvii. p. 40. 



2 Bonney : "On Serpentine of Ayrshire Coast," Q. J. G. S., November 1878. 



3 See also post, p. 388. 



4 Proc. Geol. Soc., vol. ii., 1835, p. 196. 



8 " Report and Transactions of the Devonshire Association for the Advance 

 of Science, Literature, and Art," part iv., 1865. 



