Prof. E. H'l.U — Submergence of the Isle of Wight. 67 



position, as shown b} r Mr. Codrington, 1 Prof. Prestwich, and other 

 writers, which is far beyond the reach of the rivers past or present ; 

 because of their stratified structure and their composition. 

 There remain the two other hypotheses, both equally out of the 

 question — that they are lake deposits, or that they are of glacial 

 origin. The absence of shells in the higher levels, reaching to 

 300—500 feet, is easily explained, and is no argument agninst marine 

 conditions of deposition, as the percolation of subrenal waters through 

 such open and porous beds must necessarily have dissolved away the 

 material of which marine organisms are mainly formed. As regar > 

 the geological age of the plateau-gravels of the Isle of Wight and 

 Dorset, there can be little doubt that they are post-Tertiary, as they 

 are found resting indiscriminately on all Tertiary strata of those 

 parts. 2 Prof. Prestwich classes them under the head of ; ' Southern 

 Drift," and distinguishes them from his " Westleton Beds " on the 

 ground mainly of difference in the composition of their constituent 

 pebbles, and their separation from the Westleton Beds of the East of 

 England and the Thames Basin by a barrier along the Wealden 

 ridge. But even admitting this separation, the beds may be repre- 

 sentative as regards age ; and as for the absence or presence of 

 certain pebbles (such as white quartz, etc ), it can scarcely be 

 expected that in any beds of conglomerate of wide extension there 

 should be no variation in the composition of the constituent pebbles. 

 Perhaps, on reconsideration, Prof. Prestwich might be inclined to 

 admit that the plateau-gravel of the south and the Westleton gravel of 

 the centre and east may be contemporaneous. With these preliminary 

 observations, I proceed to the evidence of submergence and emergence 

 afforded by these beds in the Isle of Wight. 



As ably shown by Mr. Codrington, the plateau-gravel of the 

 Isle of Wight originally formed a surface sloping from the south 

 northwards towards the shore of the Solent. Subsequent denudation 

 has largely modified the original range of this plateau ; wide valleys, 

 Buch as that of the Eiver Medina, have been channelled through it, 

 and in many places only outliers have been left, of which the most 

 important is that of Headon Hill. 3 In a word, enormous changes 

 in the physical features of the island have occurred since this wide- 

 spread sea-bed was laid down, which were brought about during 

 emergence in the first place, and afterwards by subaerial agencies 

 down to the present day. 



Of the numerous disconnected patches of the plateau-gravel. I 

 shall only refer specially to two — that of St. George's Down, south 

 of Newport, and Headon Hill, overlooking Alum Bay at the western 

 extremity of the isle. 



St. George's Down. — This terrace is situated in the very centre of 

 the isle, a little to the south of the Chalk ridge. Its upper sir. : 



1 Quart. Journ. Geol. Soc. vol. xxvi. 1S70. 



2 Unless it should be assumed that they are of Pliocene age, of which there is no 

 evidence or probability. 



3 The outlying patches of the plateau-gravel are shown on the new Drift map> 

 of the Geological Survey above referred to. 



