Vol. 50.] OLDEE^EEAGMENTAL EOCKS IN N.W. CAEBNAEVOKSHIEE. 579 



tremendous disturbances. The uplifting and denudation of the 

 earlier strata, previous to a renewal of deposition, are changes 

 which we should expect to find associated with a period of mountain- 

 making. This can be no local phenomenon ' : its record must extend 

 over a wide area. Yet nowhere in the surrounding districts do we 

 find evidence of such a break throughout the whole of the Cambrian 

 and Ordovician periods — unless we concede that Mr. Blake has estab- 

 lished the existence of one, farther north, immediately before the 

 beginning of the Arenig epoch. Of even this hypothesis, however, 

 he declines to avail himself here, 2 so that a second great break must 

 be introduced. Yet, so far as we know, there is no certain evidence 

 of the existence of either. Only when we arrive at the commence- 

 ment of the Upper Llandovery age have geologists agreed in recog- 

 nizing one comparable with this in importance. This difficulty is 

 dismissed in a few inconclusive sentences. Yet it is one which no 

 detailed observation, no withdrawal of any small part of the evidence 

 can possibly meet. It is general, and must be dealt with before the 

 hypothesis can have any solid foundation. 



The other difficulty, which is not even considered, is partly con- 

 nected with the preceding one. It is the necessity of twice un- 

 covering the felsite 3 in order to obtain from it a great amount of 

 material to help so largely in forming the two series of strata, the 

 one before, the other after, the tremendous break we have men- 

 tioned. 4 So complete was the exposure of the felsite in what he 

 takes for the second period that Mr. Blake describes this ' post- 

 Llanberis' conglomerate ae shading into the 'reconstructed' felsite. 5 

 But the denudation cut even deeper, since the so-called later conglo- 

 merate includes granitoid and quartzite-pebbles, which must have 

 been obtained from early pre-Cambrian rocks. Yet, curiously enough, 

 the ' Llanberis ' strata, though they have been so completely planed 

 down, have not contributed any large amount of fragments. Only 

 exceptionally do we find these or slaty pebbles of any kind, as, for 

 example, at Moel Tryfaen. The detailed evidence, then, ought to be 



1 Mr. Blake says it has to be decided whether ' this unconformity is any 

 more than a local one' (op. cit. p. 465). But surely, were it local, this would 

 require such exceptional and extraordinary conditions as to be practically 

 impossible. 



2 Ibid. p. 465. The unconformity which has been claimed in some previous 

 papers, as separating the Cambrian from the pre-Cambrian, would not of course 

 help Mr. Blake ; and this presents no serious physical difficulties. 



3 We refer to the well-known mass which is shown in the Geological Survey 

 map as near Llyn Padarn, and as extending for a considerable distance from 

 Bethesda to the S,W. 



4 Although the so-called earlier ' Llanberis Beds ' generally are finer-grained, 

 they can be clearly proved to be at places mainly composed of material derived 

 from the quartz-l'elsite. As examples we may mention beds in some of the 

 ' banded slates' synclinal east of Llyn Padarn, and the ' more felsitic material 

 .... almost .... pure felsitic ash,' described by Mr. Blake south of the 

 second conglomerate (op. cit. p. 445). 



5 Ibid. p. 447. Thus, apart from other objections, Mr. Blake's hypothesis of 

 several successive felsites (e. g. Moel Goronwy) and their denudation will not 

 lessen the difficulty referred to above, since here (and at other places) the ' post- 

 Cambrian' conglomerate is supposed to be deposited on one of the earlier felsites. 



Q. J. G. S. No. 200. 2 s 



