578 PROF. T. G. BONNET AND MISS C. A. EA1SIN ON [Nov. 1 894, 



38. On the Relations of some of the Older Fragmental Rocks in 

 North-western Caernarvonshire. By Prof. T. G. Bonnet, 

 D.Sc, LL.D., F.R.S., F.G.S., 1 and Miss Catherine A. Raisin, 

 B.Sc. (Read June 20th, 1894.) 



Contents. 



Page 



I. General Considerations ,578 



II. Evidence from Moel Try faen 580 



III. West of Llyn Pa darn 581 



IV. East of" Llyn Padarn : Inland Sections 586 



V. East of Llyn Padarn : Alleged Unconformity on the Slate Railway 588 



VI. Stratigraphical Succession 593 



VII. Comparison of Microscopic Sections 597 



1. General Considerations. 



In a recent paper published in the Quarterly Journal of the 

 Geological Society, 2 a new and revolutionary hypothesis is put 

 forward as to the age and position of certain well-known con- 

 glomerates and associated beds in North-western Caernarvonshire. 

 The new explanation presents at once some difficulties, but these 

 have been increased in number and gravity by a fresh study of the 

 district. We selected those portions of it examined by Mr. Blake, 

 in which the more critical sections occur, and tested with the 

 microscope the questions raised by work in the field. The one or 

 the other author was already in possession of a fair number of slides 

 from the district in dispute, and nearly fifty additional specimens 

 have been expressly prepared for the present paper. 



According to the new hypothesis, the strata previously mapped 

 and described from near Llyn Padarn and from neighbouring 

 districts can be distinguished into two groups separated by a 

 strongly marked unconformity. While the one part is held by 

 Mr. Blake to be of early Cambrian age, the other is ' post-Llanberis.' 

 A question at once suggests itself, namely, to what epoch (from the 

 Menevian onwards) do these so-called ' post-Llanberis ' sediments 

 belong, and where in adjacent districts may we find beds that can be 

 correlated with them ? Of this problem Mr. Blake has not succeeded 

 in offering a solution, but he evidently perceives that it is one which 

 claims consideration. 



The new hypothesis, however, would involve two consequences of 

 far-reaching import. The unconformity is described as being marked 

 by nearly horizontal beds overlying earlier strata approximately 

 vertical in position. Such an unconformity would be the record of 



1 The larger share of the work in this paper has been done by Miss Raisin. 

 She has visited all the sections, often more than once. There are some which I 

 have not examined, and others which I have not seen since the date of my 

 former paper. Tt is only just that I should make this clear. — T. G. B. 



2 ' On the Felsites and Conglomerates between Bethesda and Llanllyfni, 

 North Wales,' Quart. Journ. Geol. Soc. vol. xlix. (1893) pp. 441-465. 



