

PERMIAN VOLCANOES OF SCOTLAND. 311 



shire. The rock has a dull, coarse-grained porphyry base, in which are 

 scattered triclinic felspar and sometimes orthoclase. The base varies 

 from dark chocolate or purple to pale yellow or white, and may be 

 greenish or bluish. It is frequently amygdaloidaL Porphyrites are 

 relatively less heavy than basalts. The microscope shows the ground 

 mass to be a clear colourless, felspar, so that about nine-tenths of a 

 typical porphyrite is felspathic. Next in abundance are octahedra 

 of magnetite. Augite is not always present. 



Felstones. Finally, there are the orthoclase felspar rocks, termed 

 felstones. They consist of a finely granular felsitic ground mass, with 

 grains of quartz and crystals of orthoclase. Felstone is well seen on 

 the shore at Largo in Fife, where it forms a volcanic neck. It is 

 also seen as a yellow quartz felsite among the Campsie Fells stretching 

 into Ayrshire ; this rock was originally a rhyolite. 1 



Permian Volcanoes. 



Permian Volcanoes of Dumfries. In the northern half of the 

 Thornhill basin in Dumfriesshire, the lower part of the Permian 

 series consists of a succession of interstratined beds of porphyrite, 

 which are lava-flows associated with beds of tuff. From Nether 

 Dalbean the rock forms swelling slopes with occasional hillocks, which 

 extend southward. A large mass of it covers the carboniferous rocks 

 at Norton Castle, and it is seen to the south-east of Townfoot ; on 

 the west it occurs in the bed of the Nith. The porphyrite is identical 

 with that of the Permian volcanic rocks of Ayrshire, varying from a 

 line-grained compact rock to a dull earthly scoriaceous rock. It con- 

 sists of plagioclase felspar with much hematite, which often replaces 

 augite or other minerals. Most of the rock is much decomposed ; 

 it is best seen at the bend of the Nitli between Drumlanrigg and 

 Carron Eidge. Another grand section occurs half-way between Gate- 

 law Bridge and Kettleton Bridge on the left bank of Campsie Water. 

 These rocks rest upon the Carboniferous series, and are covered by 

 the usual brick-red Permian sandstones. Carron Water is the centre 

 of the volcanic Permian district of Thornhill, and here some of the 

 blocks of agglomerate weigh half a ton or more. In Garroch Water 

 a thin bed of ashy breccia divides these Permian rocks from the Car- 

 boniferous series. 2 



Permian Volcanoes of Ayrshire. In Ayrshire many necks of 

 volcanic agglomerate mark the site of ancient volcanoes. The coal- 

 workings have shown that they descend vertically, and destroy the 

 coal-seams for some distance around them. Five such necks are seen 

 to the south-east of Symington, three to the east of Irvine, three 

 more near Stevenstone, besides many others. Several of these necks 

 occur along a line of fault. 3 



1 Geikie : "Carboniferous Volcanic Rocks of the Firth of Forth," Trans. Roy. 

 Soc. Edin., vol. xxix. p. 437. See also Mem. Geol. Surv. East Lothian, 1866 ; 

 Geol. Edinburgh, 1861 ; and East Berwickshire, 1863. 



2 Arch. Geikie : Mem. Geol. Surv. Scotland, Explanation Sheet 9, 1877. 



3 Mem. Geol. Surv. Scotland, Explanation Sheet 22, Arch. Geikie. 



