Rev. J. F. Blake — The Llanheris Unconformity. 175 



phenomenon that I propose to see whether the supposed break is 

 local or not by way of testing its existence. 



The next difficulty is " the necessity of twice uncovering the 

 felsite," which, they say, I have "not even considered." Certainly 

 I liave not. Why should not felsite be uncovered twenty times in 

 the course of its history ? As I do not understand what the difficulty 

 is, I may be wroug iu supposing it to result from a mixture of ideas. 

 It may be their idea that there is in this district an unconformable 

 conglomerate at the base of the Cambrian series, and it is my idea 

 that there is an unconformity above that series. I do not hold both. 

 As to all the conglomerates but one, I accept Professor Hughes' and 

 Sir A. Geikie's explanations as to how a contemporaneous lava-flow 

 may be denuded before it is covered up by any other stratum, and 

 thus yield its pebbles to an overlying conglomerate. It is only 

 when the conglomerate has to lie on the felsite unoonformably that 

 the latter wants uncovering. 



The next difficulty is thus expressed: "Curiously enough the 

 Llanberis strata, though they have been so completely planed down, 

 have not contributed any large amount of fragments ; only ex- 

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

 example, at Moel Tryfaen." Here the authors give themselves 

 completely away. If there be a single pebble, exceptional or other- 

 wise, really deposited with the conglomerate and derived from the 

 Llanberis strata, then the former must be younger than the latter. 

 Probably the authors do not mean what the words iniply, but for 

 "these" we must understand "pebbles resembling these, but not 

 really derived from them." Even then the statement reads curiously 

 with reference to Moel Tryfaen, after Professor Bonney's description 

 in C, that about three-quarters of the pebbles on the western side 

 are " slate, green and purple." etc., as quoted above, while " on 

 the eastern side the fragments of purple slate are rather more 

 numerous." He then believed in the unconformity. The statement, 

 however, that the Moel Tryfaen conglomerate is exceptional in 

 containing slate pebbles is based on defective information. The 

 slopes of Y Bigl show, in certain parts of the conglomerate, quite 

 a number of purple slate fragments, of such large size and so 

 angular that I really cannot suppose them to have come from any 

 distance ; and the workable slates close by are just like thein. They 

 are found also on Mynydd-y-cilgwyn and between Moel fihiwen 

 and Moel-y-ci. We must remember, too, that purple slate is not 

 a kind of rock likely to be left in large fragments far from its source 

 of origin ; it would soon break down into mud and form a new rock 

 like the original. Of this we find several examples, which render 

 the stratigraphy difficult. 



So far from the alleged absence of pebbles of rocks like the 

 adjoining ones being a difficulty, their frequent presence and their 

 distribution are strongly in favour of an unconformity, as formerly 

 argued by Miss Raisin. It is these slate pebbles that distinguish, 

 when present, the great conglomerate from others which are intra- 

 formational. Sir A. Geikie proposed to account for them by the 



