Vol. 6o.l THE LLYX-PADARX DYKES. 373 



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upon the signs which the intrusions exhibit of having been con- 

 siderably modified by earth-pressures, more especially in those 

 portions which protrude into the Cambrian strata. Petrographical 

 considerations, also, make it impossible to separate these rocks from 

 the diabase-sills of Bala age occurring farther to the south and 

 south-west of this area ; and there is a strong presumption that 

 they represent the last residuum of the magma from which the Bala 

 sills were derived. 



II. Field-Evidence. 



The greater part of the basic dykes of .Llyn Padarn have a south- 

 easterly strike, and several of them penetrate both the older ridge 

 and the later Cambrian strata which abut upon it. Mr. Harker 

 has given very strong reasons for supposing that the ridge stood up 

 as a more or less firm buttress, against which the Llanberis Slates 

 and Grits were forced by the great south-easterly thrust which took 

 place after the commencement of the eruption of the Bala diabases. 1 

 The duration of this thrust is uncertain, but there does not appear 

 to have been, in this area, any later movement of a magnitude 

 sufficient to cause such a structural alteration as these rocks exhibit 

 in certain parts. 



A careful examination of these dykes discloses the fact that 

 whereas the north-western portions, which are enclosed in the older 

 rocks of the ridge, are comparatively free from dynamic meta- 

 morphism, this character gradually disappears as the dykes are 

 followed into the more yielding Cambrian grits and slates, where 

 they become structurally deformed, and often so highly sheared as to 

 become with difficulty recognizable as portions of the same dyke. 

 This feature is not confined to a few instances. It is shown in every 

 case that has come under my notice. Taking, for example, the long 

 dyke shown in the map (p. 376) south of Cwm-y-glo, this rock 

 preserves the character of a typical ophitic diabase, until near Gallod 

 it emerges into the Cambrian sediments. Here its course changes, 

 and it shows a curve concave to the south, as it winds upward to the 

 summit of Y Bigil. At the same time, the appearance of the rock 

 alters, its original structure being altogether obliterated by crushing, 

 and its sheared end has acquired an almost slaty cleavage. 

 Accompanying this structural alteration the mineralogical changes 

 are no less pronounced, as will be more fully detailed in another 

 section of this paper. 



Similar features may be noticed in tracing the other dykes in 

 the two areas east and west of Llyn Padarn. The dykes in the 

 ridge on the western side of the lake show only the effects of 

 slight shearing and pressure-metamorphism, while those in the 

 sediments on the eastern side, about Fachwen and Yr Alt Wen, 

 are crushed almost beyond recognition. Xot a single example of 

 the many exposures of ' greenstones ' in the Cambrian sediments, 



1 'Bala Volcanic Series of Caernarvonshire' [Sedgwick Prize Essay for 

 1888] 1889, p. 114. 



