AN^'IVEKSARY ADDKESS OF THi: PRESIDENT. 95 



margin, where it has been exposed to the pressure from the south- 

 east, the quartz-porphyry has been so crushed that it passes here 

 and there into a fine unctuous shite or almost a schist. Nowhere 

 can this change be more clearly seen than on the slopes of Mynydd y 

 Cilgwyn. The cleavage-planes strike about N. 40° E., with an in- 

 clination to dip towards the N.W. Within a space of a few yards a 

 series of specimens may be collected showing at one end an ordinary 

 or only slightly sheared quartz-porphyrj^ with abundant quartz-blebs, 

 and at the other a fine greenish scricitic slate or phyllite, wherein 

 the quartz has been almost entirely crushed down. Lines of 

 shearing may be detected across the breadth of the porphyry ridge, 

 each of them coinciding with the prevalent trend of the cleavage. 

 Sometimes also the basic dykes, which traverse the porphj'ry in 

 some numbers, have undergone considerable deformation from the 

 same cause. Their thinner portions are so well cleaved that at the 

 first glance they might quite easily be mistaken for included bands 

 of green slate. But these cleaved branches may sometimes be 

 traced into a thicker and more solid dyke, whose uncrushed cores 

 still preserve the original character of the rock and prove it to be 

 eruptive. 



The rocks which succeed the porphyry in the Valley of Llanberis 

 are of great interest, for they contain abundant proof of contem- 

 poraneous volcanic activity, and they show that, so far from there 

 being any marked hiatus here, there is evidence of the persistence of 

 eruptions even in the time of the Llanberis Slates. Considerable 

 misapprehension has arisen from the attempt to make the conglome- 

 rate the base of the Cambrian series, and the real significance of 

 the volcanic detrital strata in association with it has been missed. 

 The conglomerate does not lie on one definite horizon. In truth 

 there are several zones of conglomerate, each with some difference of 

 composition, thickness, or extent *. These may be well studied both 

 on the south and the north side of the porphyry ridge at the lower 

 end of Llyn Padarn. They are intercalated among fine tuffs, ashy 

 grits, vvolcanic breccias, and purple slates, sometimes full of fine 

 ashy material. On the south-east side of the ridge, where the rocks 

 have suffered intense cleavage, they assume a fissile unctuous 

 character, and then resemble parts of the cleaved tuffs in the 



* I could find no evidence of unconformability beneath t)ie conglomerate. 

 The section described by Professor Grreen, Quart. Journ. Geol. Soc. vol. xli. 

 (1885) p. 74, is explicable, I think, by the difTerence between the effects of cleavage 

 on the fine tuffs and the more massive resisting conglomerate which overlies them. 



