Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/310

216 Mr. objected to all the fossils attributed to the Postglacial period being regarded as synchronous, and to the cave- and river-deposits being classed together.

Mr. hoped that at some time a chronological arrangement of British caves might be proposed. He mentioned the discovery of a palæolithic flint implement in a brickfield at Highbury, and argued against the lower-level deposits of the Thames valley being regarded as more ancient than the higher.

Mr. called attention to the absence of river-gravels and caves in the Silurian region of Wales and of the North, which was owing mainly to the absence of limestone adapted for the formation of caves and of material for gravels. Prof. argued that caves such as those in which mammalian remains occur must have existed in preglacial time, and therefore that it would be strange if none of those explored contained preglacial remains. He was not satisfied as to the cause of the Thames forming a line of demarcation marking the absence of Glacial deposits. It could only be accounted for in his mind by the valley of the Thames having been entirely excavated since the Glacial period, though he acknowledged this was a bold speculation. He had always regarded the Thames-valley deposits as postglacial.

Mr. thought that the brick-earth of the lower part of the Thames valley was one of the later deposits in that valley, and therefore Postglacial. Beneath the Corbicula-bed of Crayford there were shells of some of the common living species of the neighbourhood. He saw no such extreme difficulty in the excavation of the Thames valley since the deposition of the Boulder- clay.

Mr. mentioned the Helix ruderata and H. fruticum as being instances of shells of northern character occurring in the Thames valley at Ilford. No shells of an arctic or boreal character occurred in the South of England; so that it would appear that it had not been submerged during the Glacial period.

Mr. was glad to find that the opinion of the Thames-valley deposits being Postglacial was gaining ground. He called attention to the existence in France of river-gravels belonging to an earlier period, such as that near Chartres; so that such might exist elsewhere. He could not reconcile the occurrence of Hippopotamus so far north as Leeds with its annual migration, as had been suggested.

Mr. agreed with the view of the author of the 'Reliquiæ Diluvianæ,' that the animals whose remains occur in caves lived prior to the submergence which filled the caves, or, in other words, to the Glacial period. He thought that it was impossible for all the animals whose remains occurred in the River-gravels to have occupied the land surface at the same time. He considered that English geologists were too prone to argue from phenomena confined to this country. A long island must have existed where now is the South of England at the Glacial period; but he thought that at that time all animal life must have ceased there. If so, our divisions of time could not apply to the Continent, where no such extreme changes in conditions took place.