202 J. S. GARDNER ON THE LOWER LONDON TERTIARIES. 



was deposited. The waves probably acted then on a chalk coast 

 precisely as they do now. As they undermine a cliff, the face 

 tumbles away ; and the water abrades and dissolves the chalk, leaving 

 the flint to be rolled and drifted into bays, or onto shingle banks 

 out to sea. The flint in any case ceaselessly travels, and is eventually 

 ground into mud or preserved as pebbles. 



Below the line of shingle beach we generally have between tides 

 a great stretch of water worn uneven-surfaced chalk covered and pro- 

 tected by fucoids and Laminarice ; and this extends as far out below 

 low-water as the depth permits us to see. No chalk-ooze seems 

 ever to be reconstructed ; and the carbonate of lime must therefore 

 be carried away in solution. A great deal of the chalk bottom 

 would apparently remain bare, as it is over large areas of the Channel, 

 if no other sediment were brought to it ; but when sediment is brought, 

 where the sea is shallow enough to permit the growth of sea-weed, 

 it must be originally deposited on such a seaweed-covered surface. 

 The Thauet-beds fauna shows that they must have been deposited 

 within the depth to which the Laminarian zone extends ; and this 

 imbedded sea-weed may well account for the peculiar character of 

 the " blackish green mud-like sediment in which the green-coated 

 flints are imbedded." Prestwich * states that in burning it gives 

 off ammonia in abundance, an evidence, he considers, of the presence 

 in it of animal matter. This bottom-bed might of course beloug to 

 a very much older period than the rest of the Thanet Beds. 



There is not the slightest evidence, however, that any elevated 

 chalk coast-line ever came into contact with the waves that deposited 

 the Thanet Beds in England. There are no beaches throughout the 

 Eocene with a proportion of angular flints such as we invariably see 

 when flint beaches are immediately derived from the chalk ; nor are 

 there any indications of old chalk cliff- lines. On the other hand, 

 the relatively small size and the completely rounded character of all 

 the pebbles show that they must have travelled a long way from 

 their source, and that they may have formed part of still older 

 deposits. The fluviatile deposits show that no chalk was being cut 

 through by the river, and the marine deposits that no chalk was 

 being planed down on this area by the sea. The chalk cliffs of the 

 Eocene coast-line must therefore at all our British Eocene periods 

 have been far distant. 



Above this " base-bed " we have about 19 feet 6 in. of yellowish 

 drab sandy clay, which, though of nearly equal consistence, weathers 

 out in bands, and on this some 16 feet of bluish slightly mottled 

 sandy clay with decaying pyrites. The beds here become fossiliferous, 

 and continue to be darkish sandy clay for another 34 feet, when we 

 reach the first distinct line of drifted shells. 



The shell-beds range through a thickness of about 6 feet, and 

 enclose a band of somewhat calcareous sandy concretions ; 4 feet 

 higher there is a band of scattered black pebbles ; and the section 

 closes with some 12 feet of sand. So far the section lies in Pegwell 

 Bay. The whole of the Eeculver Thanet Beds, as limited by Prest- 

 * Prestwich, Quart. Journ. Geol. Soc. vol. viii. p. 246. 



