Ch. XVI] PLASTIC CLAYS AND SANDS. 221 



proofs of tlie river and the sea having successively prevailed on the same 

 spot. At New Charlton, in the suburbs of Woolwich, Mr. De la Conda- 

 mine discovered in 1849, and pointed out to me, a layer of sand asso- 

 ciated with well-rounded flint pebbles in which numerous individuals of 

 the Cyrena telUnella were seen standing endwise with both their valves 

 united, the posterior extremity of each shell being uppermost, as would 

 happen if the mollusks had died in their natural position. I have de- 

 scribed* a bank of sandy mud, in the delta of the Alabama river at 

 Mobile, on the borders of the Gulf of Mexico, where in 1846 I dug out 

 at low tide specimens of living species of Cyrena and of a Gnathodon^ 

 which were similarly placed with their shells erect, or in a position 

 which enables the animal to protrude its siphon upwards, and draw 

 in or reject water at pleasure. The water at Mobile is usually fresh, 

 but sometimes brackish. At Woolwich a body of river water must 

 have flowed permanently into the sea where the Cyrence lived, and 

 they may have been killed suddenly by an influx of pure salt water, 

 which invaded the spot when the river was low, or when a subsidence 

 of land took place. Traced in one direction, or eastward towards 

 Heme Bay, the Woolwich beds assume more and more of a marine 

 character ; while in an opposite, or southwestern direction, they become, 

 as near Chelsea and other places, more freshwater, and contain Unio^ 

 Paludina^ and layers of lignite, so that the land di'ained by the ancient 

 river seems clearly to have been to the southwest of the present site of 

 the metropohs. 



Before the minds of geologists had become familiar with the theory of 

 the gradual sinking of land, and its conversion into sea at different pe- 

 riods, and the consequent change from shallow to deep water, the fresh- 

 water and littoral character of this inferior group appeared strange and 

 anomalous, j^fter passing through hundreds of feet of London clay, 

 proved by its fossils to have been deposited in deep salt water, we arrive 

 at L/eds of fluviatile origin, and in the same underlying formation masses 

 of shingle, attaining at Blackheath, near London, a thickness of 50 feet, 

 indicate the proximity of land, where the flints of the chalk were rolled 

 into sand and pebbles, and spread continuously over wide spaces. Such 

 shingle always appears at the bottom of the series, whether in the Isle of 

 Wight, or in the Hampshire or London basins. It naay be asked why 

 they did not constitute simply narrow littoral zones, such as we might 

 look for on an ancient sea-shore. In reply, Mr. Prestwich has suggested 

 that such zones of shingle may have been slowly formed on a Large scale 

 at the period of the Thanet sands (C. 3, p. 208), and while the land was 

 sinking the well-rolled pebbles may have been dispersed simultaneously 

 over considerable areas, and exposed during gradual submeigence to the 

 action of the waves of the sea, aided occasionally by tidal currents and 

 river floods. 



Thanet sands (C. 3, p. 208).— The mottled or plastic clay of the 



* Second Visit to the United States, vol. ii. p. 104. 



