47 1 Royal Institution. 



species, such as Cyrena cunei/ormis, &c, agreeing with fossils which 

 characterize the Lower Eocene beds at Woolwich. At Lewisham 

 the pebble-bed passes under the London Clay, and at Shooter's Hill 

 this clay overlies it in great thickness. 



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

 mine discovered a few years ago a layer of sand in the midst of the 

 pebble-bed, where numerous individuals of the Cyrena tellinella 

 were seen standing endwise, with both their valves united, the pos- 

 terior extremity of each shell being uppermost, as would happen if 

 the mollusks had died in their natural position. Sir Charles Lyell 

 described 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, he 

 had dug out, at low tide, specimens of a living species of Cyrena, 

 and of a Gnathodon, which were similarly placed, with their shells 

 erect, a position which enables the animal to protrude its siphons 

 upwards, and draw in water to lubricate its gills, and reject it when 

 it has served the purposes of respiration. The water at Mobile is 

 usually fresh, but sometimes brackish. Sir Charles examined lately 

 the Woolwich beds with Mr. Morris, and they verified Mr. De la 

 Condamine's observations, observing there several dozen specimens 

 of the Cyrena tellinella in an erect position. From this circumstance 

 the Lecturer infers, that a body of fresh or river water had been 

 maintained permanently on that spot during the Eocene period, and 

 the presence of rolled oysters in the associated pebbly layers, with 

 other marine shells, mixed with species of Melanopsis, Melania, Ce- 

 rit hium and Neritina, demonstrate that the sea occasionally invaded 

 the same area. To an overflow of the pebbly sand in which the 

 Cyrense lived by salt water, may probably be attributed the poisoning 

 of the mollusks which left their shells uninjured on the spot where 

 they had lived. 



The stratum called " the shell-bed," which contains at Greenwich, 

 Woolwich, Upnor near Rochester, and other places, a great mass 

 of freshwater, brackish-water and marine shells, especially oysters, is 

 observed everywhere to underlie the great pebble-bed. Its mode 

 of occurrence implies the entrance of one or more rivers into the 

 Eocene sea in this region. Other rivers draining adjoining lands 

 are indicated by a similar assemblage of fluvio-marine fossils near 

 Guildford and at Newhaven in Sussex. The vicinity of land to the 

 south and west of Woolwich is shown by the occurrence at New 

 Cross, Camberwell, and Chelsea of Puludina and Okie in strata 

 evidently a prolongation of the Woolwich beds, and by fossil leaves 

 of dicotyledonous trees and la}'ers of lignite in some of those loca- 

 lities. On the other hand, at the junction of the " London Clay," 

 and the subjacent " plastic clays and sands," when followed in an 

 opposite or easterly direction towards Heme Bay and the Reculvers, 

 all signs of the freshwater formation disappear, and the pebble- bed 

 is reduced to a thin layer, often a foot or a few inches in thickness. 

 The origin of this shingle may have been chiefly due to the action of 

 waves on a sea-beach. Its accumulation in great force at certain 

 points where freshwater shells abound, seems to imply the entrance of 



