G. W. Tyrrell— Intrusions of Kihyth-Croij District. 299 



8. Shipman, J. Manuscript notes, in University College, Nottingham. 



9. Davis, J. "W. Quart. Journ. Geol. See, vol. xxxii, p. 332. 



10. Gibson, W., and others. "Summary of Progress for 1906": Mem. Geol. 



Suiv., p. 11. 



11. Hind, W. "Monograph on Carhonicola, Anthracomya, and Naiadites,^'' 



1895, pt. ii, Section 7 : Pal. Soc, vol. xlix. 



12. Strangways, C. Fox. " Geology of the Leicestershire and South Derbyshire 



Coalfield," 1907 : Mem. Geol. Surv. 



II. — Geology and Petkology of the Intrusions of the Kilsyth- 



Croy District, Dumbartonshire. 



By G. AV. Tyrrell, A.R. C.Sc, Assistant in Geology, Glasgow University. 



(PLATE XII.) 



I. Introduction. 



THE group of intrusions of which this paper treats is a part of 

 a series of intrusive rocks piercing the Carboniferous strata of 

 the Midland Valley of Scotland. As a rule, they have forced 

 their way into the Carboniferous Limestone Series in layers roughly 

 parallel to the stratification. At the surface they stand out as rough, 

 craggy hills, often of considerable elevation, rising abruptly out of the 

 plain of the Carboniferous sediments, and frequently ending ofp in 

 a vertical escarpment of bare I'ock. They generally confront, at 

 a distance of 1 or 2 miles, the terraced escarpments of Lower 

 Carboniferous lavas, which bound the Midland Valley on the north, 

 west, and south-west. 



These intrusions are intimately connected with the great series of 

 E.-W. dykes which traverse the Midland Valley, and form, with 

 them, a group of considerable petrographical individuality, which is 

 represented in all pjarts of the world. 



On the ground the Kilsyth-Croy group consists of four separate 

 masses, extending roughly in a S.W.-N.E. direction. It is connected 

 with three of the great E.-W. dykes. ^ 



The two main outcrops, upon which stand the villages of Kilsyth 

 and Croy respectively, extend from Twechar to Banton, a distance of 

 5 miles, and are separated by the ancient alluvial trough now occupied 

 by the River Kelvin and the Forth and Clyde Canal. This trough, 

 however, cuts through the north-eastern prolongation of the Croy mass. 

 The third outcrop is a thin, sinuous intrusion, about 1 mile south of 

 the Croy mass, which is injected into a higher horizon. Another 

 small mass occurs abutting against the main boundary fault to the 

 north of Kilsyth. 



A short general account of the nature and occurrence of these rocks 

 is given in the Geological Survey Memoir (1879) on Sheet 31.^ In bis 

 classic paper on the Carboniferous Dolerites,^ Allport described the 

 rocks of Gernal Brae and Auchinstarry in the Kilsyth district. Sir 

 A. Geikie has treated of the general relations of these intrusions and 

 the great E.-W. dykes, first in his paper on the Carboniferous Volcanic 



1 See Sheet 31 of Geological Survey of Scotland. 



■ pp. 43-6. 



3 S. Allport, Q.J.G.S., 1874, vol. xxx, p. 553. 



