The present paper deals with some 30 square miles of land situated in Caernarvonshire and embracing the drainage area of the River Ogwen and parts of adjacent river-basins. It is doubtful if any part of Great Britain presents in such a small area so many interesting topographical features and such beautiful and diversified scenery as this part of North Wales. It is in part a thoroughly mountainous region accompanied by characteristics that belong to mountains, and it is a glaciated mountain region with typical glacial topography. But adjoining the mountains is an area of entirely different characteristics, and the change from the one to the other is sudden and complete. An upland plain abruptly terminates against a range of mountains without the interposition of foot-hills, the crags and pinnacles rising precipitously from the level land; or, in other words, the plain cuts as it were a shelf in the mountainous masses.

Since the earlier edition on the geology of Kinta was written much fresh evidence has been brought to light on the subject of the origin of the clays and boulder-clays and the tin-bearing deposits showing bedding at Gopeng. The effect of this evidence has not been to lessen the objections to the glacial hypothesis put forward by myself, but at the same time it still remains the only explanation that meets the facts in a way that can be called at all satisfactory. It may be that long acquaintance with the subject has made me see difficulties in the way of other explanations where in fact no difficulties exist, and my position regarding the question is somewhat akin to that of a doctor versed in tropical medicine who once informed me that the result of many years study of the etiology of beri-beri was that he felt he could raise fatal objections to any theory that had been proposed. I have not seen sufficient reason as yet, however, to change my views on the subject of these clays and boulder-clays. Certain sections to be noted later militate against a glacial origin, but the evidence of these deposits, including those showing bedding at Gopeng, being older than the granite of the Main Range is stronger than it was before. In the following paragraphs I will attempt to give briefly a statement of the points for and against all possible explanations of the peculiarities observed in these important sources of tin-ore.

Among a number of Cirripede plates from the Chalk Marl and Cambridge Greensand of Cambridge submitted to me some time ago, were certain fossils which at first puzzled me considerably. Although there were more than twenty examples, all came apparently from the same side of the animal, that is, they were not left and right, and this led me to suspect that they were not Cirripede valves, and to examine them more closely. One edge close to the narrow end of the shell was then seen to be broken quite clean and straight, and on comparing these fossils with some Pelecypod shells from the same horizon it was quite clear that they were the anterior ears of right valves of Aucellina gryphœides (Sow.) (Text-fig. 7, p. 170), a shell belonging to the family Pteriidae (see H. Woods, Pal. Soc. Monogr. Cretaceous Mollusca, 1905, vol. ii, p. 72, pi. x, figs. 6–13). Other specimens submitted at various times from Jurassic and Cretaceous rocks, turned out on examination to be the anterior ears of the right valves of Pelecypod shells like Pecten, and a number of such specimens were included among some Cirripede plates from the Chalk of liugen obtained for the British Museum by Frau Agnes Laur.