304 G. W. Tyn'ell—Intnmom of KUsyth-Croy District. 



are very much the same. Sir A. Geikie ^ believes that the large 

 intrusive masses of Linlithgowshire are processes from the long dykes. 

 Thus the dyke running eastward from Cadder and Bishopbriggs in the 

 neighbourhood of Glasgow has no fewer than four large appendages, 

 which are in turn cut by younger E.-W. dykes. He also remarks 

 the petrographic identity of the rocks of the dykes and laccolites. He 

 connects these intrusions with the puy eruptions of the Carboniferous 

 Limestone period. "The remarkable group of sills between Kilsyth 

 and Stirling may quite possibly be connected with a groirp of vents 

 lying not far to the eastward, but now buried under the higher parts 

 of the Carboniferous Limestone, Millstone Grit, and Coal-measures."^ 



The revision of the Bathgate district by the Geological Survey has 

 shown that in some cases the dykes cut the sills, but that in one case 

 at least, at the Knock in the Bathgate Hills, a N.-S. intrusion, 

 continuous with a faulted sill, cuts through one of the E.-W. dykes. ^ 



The Survey are also inclined to believe that the dykes cut the sills 

 in the Kilsyth district and that they are later than the faulting.'* 



To summarize, the relations of the laccolites and dykes to each other, 

 and of both to the faulting, appear to be contradictory in different 

 parts of the Midland Valley. The difficulty is probably due to an 

 assumption of strict contemporaneity for the various intrusions, and 

 also for the faulting ; whereas it is more probable that both intrusions 

 and faulting, whilst belonging to the same general epoch, were a series 

 of events extending over a long period of time. The earlier intrusions 

 would then be shifted by the later faulting, and the later intrusions 

 would cut through the earlier faults without dislocation.* 



If the intrusions are in general contemporaneous with the faulting, 

 they belong to that great period of earth-buckling which took place 

 at the end of the Carboniferous and the beginning of the Permian 

 period. The great E.-W. dykes cut through all the strata of the 

 Midland Yalley up to the drift, and are therefore post-Carboniferous. 

 ]!^one of the E.-W. dykes, however, are found cutting the Permian 

 Red Sandstones of Ayrshire. In all probability these intrusions were 

 a phase of the formation of the great sunken block of strata — the 

 great Rift Valley — which forms the Midland Valley of Scotland. It is 

 a striking fact that the peculiar hybrid petrographic type, granophyric 

 diabase, which forms these intrusions, very frequently occurs in these 

 long massive dykes and their accompanying protrusions in connection 

 with block faulting. 



A mass of evidence has now accumulated in support of the late 

 Palaeozoic age of these intrusions. Whilst Sir A. Geikie connects the 

 great group of sills from Milngavie by Kilsyth to Stirling with the 

 Carboniferous Limestone puys, he classes the Linlithgowshire group, 

 connected with the more southerly E.-W. dykes, as Tertiary.^ As 

 before stated, the feeders of the Kilsyth-Croy laccolite appear to cut 



' Ancient Volcanoes of Great Britain, 1897, vol. ii, p. 157. 



■ Ancient Volcanoes, vol. i, p. 447. 



3 Snmm. Prog. Geol. Surv., 1904, p. 118. 



* Ibid., 1905, p. 124. 



^ See Ancient Volcanoes, vol. ii, p. 158. 



^ Ancient Volcanoes, vol. ii, p. 10. 



1 



