Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/111

 beds. This assumption I brought forward in 1866*, when first endeavouring to show the process by which I consider the Wealden denudation to have been accomplished, in ignorance that it had been already, as I find, suggested by Mr. Mackie†. The suggestion, however, when fitted into its proper place as to time, i. e. posterior to the Thames gravel, appears to me an essential ingredient in the proposition, because the existence of such a barrier between the Channel and the North Sea must have largely augmented the tidal rush and consequent erosive action of the waters within the Weald. On the other hand, the diminution of this tidal scour, produced by the opening of the Dover Straits, supplies an efficient cause why the elevation of the Wealden area should overmaster the denuding agency, and so extricate the Weald altogether from the sea.

It may be further added that this state of things agrees with the features of the marine deposit skirting the sea between Selsey and Worthing, in Sussex, described by Mr. Dixon and by Mr. Godwin- Austen. A numerous fauna has now been obtained from this deposit by Mr. A. Bell, which, while it is quite unlike that of any of the glacial deposits, and also unlike any of the marine postglacial deposits in other parts of England, and in Scotland‡, nevertheless consists entirely of species that are still living. Nearly all the shells are denizens of our extreme southern shores ; but a few do not reach us, having their northern limit on the Lusitanian coast, so that this deposit indicates that at some Postglacial period the British Channel was subjected to an influx of Lusitanian water, which afterwards ceased and was followed by a change, under the influence of which certain Lusitanian mollusca disappeared from our shores. This order of succession is shown by the deposit in question being overlain § by a few feet of deposit containing some large angular erratics.

This overlying erratic deposit, I take the opportunity of observing, I regard as quite unconnected with the glacial beds, — its erratics being due to the presence of conditions of climate such as introduced the large angular blocks into beds of the Thames gravel series at Grays, the greywether blocks into the Postglacial gravel of Hampshire, and the boulders into the Postglacial clay of Hessle, in Yorkshire, — such deposits being due to conditions of climate wholly unlike those which gave rise to the Greenlandic conditions of the Glacial period, but similar to what now obtains in the Gulf of St. Lawrence and other parts where ice forms on coasts during the winter. The deposit, however, may perhaps indicate a colder sea-


 * Geol. Mag. vol. iii. p. 402.

† Geologist, vol. iii. p. 203.

‡ I group all the Scotch so-called Glacial shell-beds as Postglacial, as they rest on the Boulder-clay, and have a very different fauna from the Glacial beds proper, which include the Boulder-clay on which these Scotch beds rest.

§ Godwin-Austen, Quart. Journ. Geol. Soc. vol. xiii. p. 49. According to Mr. Austen the fossiliferous deposit has large portions of the skeletons of Elepkas primigenius imbedded with the shells, and is underlain by red gravel. This gravel may probably, therefore, belong to the age of the Thames beds, or nearly so.

VOL. XXVII. PART I. C