AND PEBIDIAN) EOCKS IN CAEENABVONSHIEE. 151 



been observed in other slate -quarries in these areas also where rocks 

 of unequal texture have been folded together. 



It is perfectly clear, after the evidence derived from the Moel- 

 Tryfaen area, combined with that obtained in South "Wales, that the 

 interpretation given by Prof. Ramsay, Prof. Sedgwick, and others 

 as to the intrusive characters of these porphyritic ribs is not satis- 

 factory, according to recent knowledge, and that it cannot properly 

 explain the age and succession of these beds. 



I believe there can be no doubt now that the Lower Cambrian series, 

 as far as they are anywhere known in Wales, exhibit their proper 

 thickness, and show their usual successive changes in character, 

 when not affected by faults, and comparatively in an unaltered state 

 all along this great axis of metamorphic rocks in Caernarvonshire — 

 and that the conglomerates at the base rest upon beds of this axis, 

 which are now in much the same condition as they were before any 

 of these Cambrian sediments were deposited upon them. 



The supposed intrusive nature of these ribs, and the apparent 

 passage by gradual alteration mentioned by various observers, are 

 mainly due to the fact that the matrix in the conglomerates has been 

 derived from rocks immediately below or from similar ones, and from 

 a slight subsequent change in the matrix, due, probably, to proximity 

 to the intrusive dykes, aided by a readiness perhaps in the material 

 to assume this change. This is clearly observed by watching the 

 weathering of these conglomerates even when in direct contact with 

 the porphyritic series ; for any apparent melting-away of the hard 

 pebble is shown not to be a fact, since on very slight weathering the 

 pebble becomes easily separable from the matrix, and its outline is 

 as perfect as on the day it became cemented in the mass. 



This appearance as if of a gradual passage from conglomerates to 

 rocks below, and from which most of their materials must have been 

 derived, has often presented itself to my mind in examining these basal 

 lines of the Pre-Cambrian rocks ; but a careful and minute examination 

 of the matrix and pebbles has invariably shown that such was not 

 really the case. It must not be forgotten also that in speaking 

 generally of the Cambrian rocks as an unaltered series, it is only 

 meant that they are comparatively so when in juxtaposition with 

 a true metamorphic series, such as we invariably find the Dimetian 

 and Pebidian rocks to be. The Cambrian rocks themselves suf- 

 fered some change before consolidation ; and cleavage would have 

 taken place in them ; and a state of semicrystallization is also ob- 

 servable occasionally in some of the hard gritty bands, especially 

 those associated with the lower slate beds, such as the one men- 

 tioned in the Alexandra quarry &c. This great mass or, as I would 

 now say, great area of Pre-Cambrian rocks extending through the 

 heart of Caernarvonshire from below Tal-y-sarn on the south to 

 St. Anne's chapel near Bethesda on the north, along with another 

 area near Caernarvon and Bangor described by Prof. Hughes, 

 are undoubtedly portions still remaining of the old Pre-Cambrian 

 land; and in them, I believe, are to be recognized representatives 

 of the two great unconformable series, Dimetian and Pebidian, so 



