302 G. W. Ti/rreU—Intrmions of Kikyth-Cyoij District. 



form. — The form of the Kilsyth-Croy intriisioa is that of a flattened 

 and attenuated laccolite, as is well shown in sections kindly supplied 

 by the Carron Co. and by William Baird & Co. (see Fig. 1). In the 

 jSfethercroy Pit, worked by the Carron Co., the sedimentaries and 

 the igneous rock dip to the south at an angle of 20°. The ' whin ' 

 occupies most of the surface, rising to a height of 481 feet above 

 sea-level. At its thickest part, under the Antonine Wall, it is repre- 

 sented as 346 feet thick in the section supplied by the Carron Co., 

 and this is not the original thickness, as the surface has suffered 

 considerable denudation. But at a point 780 yards south of the 

 Antonine Wall the total thickness is only 190 feet. The greatest 

 thickness in that extension of the laccolite upon which the town of 

 Kilsyth stands is about 150 feet; and although this mass has suffered 

 some denudation, it seems probable that in this direction there is 

 a considerable diminution from the maximum thickness that occurs 

 under the Antonine Wall. At Drumgrew, 2^ miles south-west of the 

 latter point, the thickness has been proved in a bore to be 108 feet. 



At the Neilston Pit, north of Kilsyth, and due north of the point 

 of maximum thickness, the igneous rock has been proved to a depth 

 of 100 feet. 



Owing to its relatively small thickness as compared with its 

 horizontal extent, the laccolite has caused a quite insignificant arching 

 of the overlying strata. 



The term ' laccolite ' is used here in the sense of a lens-shaped body 

 of igneous matter supplied through vertical dykes, both upper and 

 lower surfaces being slightly transgressive, whereas in the typical 

 laccolites of the Henry Mountains both surfaces are conformable to 

 the strata.^ 



Relations of the E- W. Dykes to the Laccolites. 



Two long E.-W. dykes terminate in the Croy mass. The more 

 southerly of these begins about 3 miles south of Linlithgow, and 

 on Sheet 31 of the Geological Survey is mapped as cutting through 

 the great sill at Torphiehen. After passing through the village of 

 Cumbernauld it is represented as cutting through the sinuous in- 

 trusion south of the Croy mass, and before terminating in the laccolite 

 throws off another small protrusion to the south. Its total length is 

 17 miles. The northerly dyke, passing through the village of Dullatur, 

 continues in a more or less parallel course to that of the Cumbernauld 

 dyke, at a distance of 1 to 1-t miles. It is also represented as 

 cutting through the Torphiehen sill, and also through the Kilsyth- 

 Croy laccolite. Its total length is 20 miles. The third dyke is only 

 2 miles long. It is represented on the Survey map as cutting 

 through the extreme south-western prolongation of the Croy mass. 



There can be little doubt but that these dykes stand in the relation 

 of feeders to the intrusions. In the Nethercroy section before cited, 

 a thick vertical dyke (100 feet) rises through the strata, and joins the 

 base of the laccolite in such a way as to leave no doubt that it is the 

 duct through which the igneous material was supplied. This dyke is 

 almost certainly continuous with the Cumbernauld. 



1 Gilbert, Geology of Henry Mountains, 1877, p. 19. 



