1870.] WOOD WEALD-VALLEY DENTTDATION. 17 



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, i n ignorance that it had 

 been already, as I find, suggested by Mr. Mackief. The sug- 

 gestion, 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 Korth 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. BeU, which, while it is quite unlike that of any of the 

 glacial deposits, and also unlike any of the marine postglacial depo- 

 sits in other parts of England, and in Scotland J, nevertheless con- 

 sists 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 over- 

 lain § by a few feet of deposit containing some large angular erratics. 



This overlying erratic deposit, I take the opportunity of ob- 

 serving, 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 cUmate 

 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. 



t G-eologist, 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. Joui-n. Geol. Soc. vol. xiii. p. 49. According to 

 Mr. Austen the fossiliferous deposit has large portions of the skeletons of 

 Eiepkas 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. PAKT I. C 



