208 J. S. GARDNER OX THE LOWER LONDON" TERTIARIES. 



when at TJpnor the shelly clays with Qyrena make their appearance. 

 The sharp sands in this section (Mem. Geol. Surv. vol. iv. part 1, 

 p. 144) I should feel inclined to place with the Thanet Beds, thus in- 

 creasing their thickness there by some 30 feet. At Erith the plastic 

 mottled clay of the Woolwich-and-Reading Beds first appears, and is 

 some 9 feet thick. At Charlton it is absent ; but at Loam Pit Hill I 

 detected it a few inches thick* in its proper position below all the 

 fossiliferous Woolwich Beds. I should most decidedly place the base 

 of the Woolwich-and-Reading Beds at this horizon, and consider all 

 the marine shingles and sands below the plastic clay an integral 

 part of the marine Thanet formation. In the magnificent section 

 recently exposed in the new railway works at Croydon, the division 

 is very clearly seen, and the whole Woolwich-and-Reading series is 

 exposed. Below the plastic clay and above the admittedly Thanet 

 beds is a series of clayey sands with green grains, of marine origin 

 and with occasional fossils and flint pebbles. There is not the re- 

 motest reason, that I can discern, for separating these from the 

 underlying series and making them a marine member of the over- 

 lying nuviatile series. As the section will probably be described by 

 Prof. Morris and Mr. Klaassen, it is unnecessary to go into details 

 here ; but if my classification is admitted there would simply be a 

 Lower Reading series with a very distinctive flora, and an Upper 

 Woolwich series with a very distinctive flora and fauna — the former 

 purely nuviatile, and the latter becoming estuarine towards its close 

 and therefore marking a subsidence with its inevitable accompani- 

 ment, an advance of salt-water conditions up the estuary. 



To meet beforehand one criticism that may possibly be advanced 

 against this view, I may at once state that I feel no doubt that 

 the marine Dorsetshire oyster- and pebble-beds hitherto referred to 

 the Woolwich beds, are of London-Clay age, and mark the western 

 shore of that sea, and not of a previous sea. The Reading Beds 

 at Alum Bay are purely nuviatile mottled clay, and the London Clay 

 rests directly on them ; but further west the London Clay is replaced 

 by oyster-beds, pebble-beds, and pyritous sandy clays resting on the 

 same mottled clays on which the London Clay rests at Alum Bay. 

 It would be rather inexplicable if the Woolwich-and-Reading series, 

 after becoming more and more distinctly fluviatile from east to west, 

 should again become marine in the furthest west ; while on the other 

 hand the London-Clay sea, which was of considerable depth as near 

 as Alum Bay, must undoubtedly have possessed a shore to the west 

 of that point. 



The Thanet Beds were probably deposited, as I have shown, by a 

 rough sea in an area out of the immediate estuary of the river, but 

 within its influence. The area became silted up until it finally 

 rose above the surface and became covered over with shingles and 

 sand. The Thanet Beds close with a period of elevation, during 

 which the Reading Beds were formed ; and these were followed by 

 a subsidence during the Woolwich period, which finally ushered in 

 the Oldhaven and London-Clay deposits. The Oldhaven Beds may 

 * Bed 9, section Loam Pit Hill, p. 127, Geol. Survey, vol. iv. pt. 1. 



