BASIN OF LONDON. 361 



They are named septaria, and are extensively used for 

 cement ; on breaking them for this purpose, some organic 

 substance, as a shell, plant, or fruit, is usually found con- 

 stituting the nucleus of the mass. The lower division is 

 composed of alternating beds of sand, shingle, clay, and loam, 

 which, from some of the clays being used for pottery, occa- 

 sioned the name of the Plastic clay formation to be bestowed 

 on the entire series. The character of their embedded 

 fossils proves the marine origin of these deposits ; consisting 

 of the teeth and vertebrae of sharks, the remains of Crustacea, 

 with those of marine mollusca and corals. The vegetable 

 reliquiae, consisting of the leaves, fruits, and stems of plants, 

 with masses of wood perforated by teredines, evince the 

 proximity of land, and show that these objects had been 

 transported by rivers and tributary streams to the then 

 existing ocean. 



THE ISLE or SHEPPET is an outlier of the London 

 clay. The beds are so strongly impregnated with iron 

 pyrites as to render their contained fossils extremely liable 

 to decomposition, and compels the collector to have recourse 

 to particular methods, as boiling them in linseed oil, or 

 keeping them in water, in order to preserve them. The 

 fruits of this locality are so abundant, that Mr. Bower- 

 bank has commenced a work devoted exclusively to their 

 illustration. They are all of a tropical nature, and constitute 

 an important feature of the eocene flora. By some this 

 district was supposed to have been the site of numerous 

 spice-islands ; by others it has been regarded as an estuary 

 deposit, and the fruits are conjectured to have been drifted 

 by a river from the land. 



Some of the characteristic shells of the London clay are 

 represented in fig. 260. 



Fig. 261 represents a piece of fossil wood, pierced by 

 TeredincB, a boring mollusk allied to the Teredo, which proves 

 so destructive to the timber of vessels. The wood is now con- 

 verted into a stony mass. " It must have once been buoyant 

 and floating in the sea, when the teredines lived upon it, 

 perforating it in all directions. But before they settled on 

 the wood, the branch of a tree must have been floated down 

 to the sea by a river, uprooted, perhaps, by a flood, or torn 

 off and cast into the waves by the winds ; and thus our 



