in the Geology of the Neighbourhood of London. 95 



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 Gnatkadon, 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 circum- 

 stance 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, 

 Cerithium and Neritiria, demonstrate that the sea occasionally invad- 

 ed 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 molluscs 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 

 fresh water, 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 shewn by the occurrence at New 

 Cross, Camberwell, and Chelsea, of Paludina and Unio in strata 

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

 of dicotyledonous trees and layers of lignite in some of those loca- 

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

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

 opposite or easterly direction towards Heme Bay and the Reculvers, 

 ail signs of the fresh-water 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 fresh-water shells abound, seems to imply the entrance of 

 rivers into the sea, which brought down some flints, and arrested the 

 progress of others travelling as beach-pebbles along a coast-line, 

 in a certain direction, determined by the prevailing currents and 

 winds. The spreading of the pebble-bed over a wide area may 

 be accounted for by supposing a gradual subsidence of land, and the 

 continually shifting of the coast-lines upon which shingle accumulated. 

 This same subsidence is required to explain the superposition of the 



