Vol. 60.] THE LLYN-PADARX DYKES. 387 



from a 'greenstone'- dyke in the Penrhyn Slate-Quarry, supports this 

 view ; but a large number of analyses of the Caernarvonshire rocks 

 would be necessary before Mr. Harker's differentiation-theory could 

 be adequately tested on chemical grounds. This theory is virtually 

 an application of Gouy & Chaperon's principle, 1 which, it is true, 

 receives some support from physical chemistry and from observa- 

 tions in the case of certain alloys. 2 



With regard to the exact time of the intrusions, it is certain that 

 the fissures were not open before the crush began, because there is very 

 little evidence of displacement in the dykes themselves. The Clegyr 

 dyke alone shows any marked sign of deflection. Mr. Harker, 

 however, mentions the occurrence of local thickening of some of the 

 dykes in the slate-quarries 3 owing to the effects of the thrust. It 

 might also be urged as an objection to the view that these fissures are 

 a result of the south-easterly crush, that their direction is approxi- 

 mately at right angles to the axes of the folds. In a perfectly- 

 homogeneous rock, pressed by uniform forces against an immovable 

 buttress, the maximum shear should be at an angle of 45° to the 

 direction of the pressure. These conditions, however, did not exist. 

 The strata were not homogeneous, the pressure was probably by 

 no means uniform, and the buttress almost certainlv Yielded more 

 or less. It is therefore quite conceivable that the buttress cracked, 

 and thus determined the direction of the fissures in the sedimentary 

 strata. 



The assumption that these dykes are of post-Carboniferous age 

 would involve two very unlikely conclusions : namely, that the later 

 magma was almost identical in its composition and in its mode 

 of consolidation with the basic injections of Bala age ; and also 

 that earth-movements of sufficient intensity to cause structural 

 deformations of parts of these dykes have operated since the great 

 south-easterly crush which folded and cleaved the slate-rocks of 

 Llanberis. Of this there is no evidence, so far as I am aware ; and 

 if such were the case, we should even then have to explain the 

 phenomena with which this paper chiefly deals, that is, that those 

 portions of the dykes which were protected by the ridge have 

 largely escaped the deformation to which their more easterly parts 

 have undoubtedly been exposed. On the other hand, all the facts 

 appear to agree with the suggestion that the Llyn-Padarn 

 fissures were injected with the last dregs of the Bala 

 magma before the effects of the post-Bala crush had 

 entirely ceased. 



EXPLANATION OF PLATE XXXII. 



[All the figures are magnified about 30 diameters. ] 



Fig. 1. Composite augite-crystal, showing crystallograpkic continuity, but 

 extinguishing in irregular areas. Crossed nicols : 1-inch objective. 



1 ' Sur la Concentration des Dissolutions par la Pesanteur ' Ann. de Chiinie 

 & de Physique, ser. 6, vol. xii (1887) p. 384. 



2 See A. Findlay ; The Phase-Kule ' 1904, chap. xiv. 



3 'Bala Volcanic Series of Caernarvonshire' 1889, p. 115. 



