G. W. Tyrrell— Intrmions of Kihijth-Ci'oy Bidi'ict. 305 



the Linlithgowsliire intrusions, and are therefore of later date. If the 

 northern group is of late Palaeozoic age, so then are the Linlithgowshire 

 intrusions. Professor Judd is of the opinion that some of the great 

 E.-W. dykes are of Carboniferous age, but some, like those of 

 Eskdalemuir and Cleveland, are undoubtedly Tertiary.^ The latter 

 may be distinguished petrographically from the Palaeozoic dykes. 

 They contain no micropegmatite, have a more basaltic structure, 

 a more acid chemical composition, and are frequently porphyritic. 

 Dr. Teall has remarked the difference between the two sets of dykes 

 in the north of England occurrences.^ The Tertiary dykes (Cleveland 

 and Acklington) have a Si Oo percentage of 57-9 and a sp. gr. '2-7-2-8. 

 The earlier dykes (Hett and High Green) have a SiOg percentage of 

 50-3 and a sp. gr. of 2-96. The latter, therefore, correspond exactly 

 with such of the dykes and connected sills of Central Scotland as have 

 been analysed.^ Kecent survey work in Scotland has added much 

 further evidence in favour of the pre-Tertiary age of the great E.-W. 

 dykes of Central Scotland.* 



IV. Petrology. 



The rocks to which the following description applies are those of 

 the Kilsyth-Croy, Stirling, and Milngavie laccolites, and the great 

 E.-AV. dykes of Bishopbriggs, Dullatur, Cumbernauld, Elanefield, 

 Whistlefield, and Rowardennan. They are all diabases containing 

 plagioclase,augite, andiron-ores, with a ground-mass of micropegmatite. 



Megascopic Characters. 



The main mass of these intrusions is composed of a grey, medium- 

 grained, phanero-crystalline rock, hereinafter referred to as the 

 'normal' type. The grey colour is due to the blending of white and 

 black minerals, the former felspars and micropegmatite and the latter 

 augite and iron-ores. The micropegmatite is not recognizable as such 

 in the normal rock, but in a coarser variety, which occurs sporadically 

 towards the centre of the mass, it appears as large pink areas, and the 

 augite tends to take on a long blady form. This rock consequently 

 closely resembles some syenites and diorites, and frequently figures as 

 such in dealers' catalogues. The marginal facies is a dense, black, 

 basaltic rock, breaking with a conchoidal fracture, and sometimes 

 porphyritic with felspar. Throughout the rock, but much more 

 frequently towards the centre of the masses, there are to be found 

 ramifying, anastomosing veins and strings, up to 2 inches in diameter, 

 of a light-coloured acid material, the so-called segregation veins. 



As a larger structural feature, the rock frequently occurs in a rude 

 columnar form, to be seen in the larger quarries and in the natural 

 escarpments. It weathers into large cuboidal blocks with rounded 

 edges and corners. In an extreme phase of weathering it breaks 

 down to a brown or red earth containing exfoliating spheroids of less 

 decomposed rock. 



' Q.J.G.S., 1889, p. 209. 



' Q.J.G.S., 1884, vol. xl, pp. 209-47, and Brit. Petrog., 1888, p. 206. 

 ^ See petrogiaphical part of paper. 



* See Memoir on Cowal district, 1897, p. 147, and Summ. Prog. Geol. Siirv., 

 1904-6, op. cit. 



DECADE V. VOL. VI. — NO. VII. 20 



