G. W. Tyrrell— Intrmiom (yf Kihyth-Cvoy Bidvid. 303 



A section kindly supplied by the Baird Co. of their workings at 

 Croy and Kilsyth also shows a 100 feet dyke rising through un- 

 disturbed strata and joining the base of the laccolite under the town 

 of Kilsyth. On the Geological Survey map the DuUatur dyke is 

 represented as cutting the Croy mass in the neighbourhood of 

 Dullatur. Inside the boundary its trend alters from due E.-W. to 

 E.S.E.-W.N. W. I have been unable to trace in the field the evidence 

 upon which this change of trend was based ; possibly it was based upon 

 underground evidence available at the time of the original survey. It 

 is significant, however, that if this change of trend be accepted, and if 

 the dyke be continued in the new direction, it would appear in the 

 position of the 100 feet dyke encountered in the Baird workings. It 

 is highly probable, therefore, that the latter and the Dullatur are one 

 and the same dyke. 



The evidence, then, seems to point to the conclusion that the 

 material of the Kilsyth-Croy laccolite was supplied through the ducts 

 represented at the surface by the Dullatur and Cumbernauld dykes, 

 and undergi'ound by the thick dykes found in the Baird and Carron 

 workings respectively. This conclusion is supported by the following- 

 facts : — 



1. The dykes terminate in the laccolite, and appear beneath it in 

 tlie position of ducts. 



2. The thickest part of the laccolite occurs between the two dykes, 

 but nearer to the Cumbernauld dyke, suggesting that the latter 

 supplied most of the material. 



3. The laccolite seems to thin away more rapidly to the north and 

 south of the point of maximum thickness, i.e. in directions at right 

 angles to the direction of the dykes, than in the opposite direction, 

 as might be expected if the dykes are the feeders of the laccolite. 



Moreover, if lines are drawn connecting points of the same 

 thickness on the laccolite, these are seen to be ellipses of which the 

 major axes are parallel to the E.-W. dykes, although the S.W.-N.E. 

 direction of extension of the laccolite itself is conditioned by the 

 anticline of the Riggin. 



4. There is complete petrographical identity between the laccolite 

 and the dykes. 



Age of the Intnmom. 



The Kilsyth-Cro}' laccolite appears to have partaken of the flexures 

 of the strata into which it is intruded, and is broken by both E.-W. 

 and jN^.W.-S.E. faults. There is no conclusive evidence in this district 

 as to the relation of the dykes to the faulting. Four miles to the east 

 of Dullatur the Dullatur dyke is mapped as uprising along a small 

 E.-W. fault. Still further to the east, at Cardrum Moss, there is 

 a slight break in the continuity of the dyke, which may possibly be 

 due to faulting. 



In the Denny district the thin sills encountered in Messrs. A . & G. 

 Moore's Castlerankine workings seem to cut through minor faults 

 M'ithout dislocation. 



The relations of the similar masses of intrusive rocks in the Midland 

 Valley of Scotland outside our immediate district to the main faulting 



