144 On some of the Strata in the Neighbourhood of London, 



tolerable specimen. With these also occur numerous Ceri- 

 thia, J^urrilellce and Cytlurece, Lam. all of which are in a 

 similar state with the oysters, and appear to be shells strictly 

 belonging to the subjacent stratum, but which, having lain 

 uppermost, became involved in the first or lowest deposition 

 of the blue clay. 



Immediately beneath the clav there is found a line of 

 about three or four inches of the preceding shells imbedded 

 in a mass of calcareous matter, the result of their disinte- 

 gration. Beneath this are numerous alternating layers of 

 shells, marl, and pebbles, for about twelve or fifteen feet. 

 The shells are those which have been already mentioned ; 

 but are very rarely to be met with whole, and when entire 

 are so brittle as to be extricated with much difficulty. In 

 some of these layers scarcely any thing but the mere frag- 

 ments of shells is to be found, and in others a calcareous 

 powder only is left. 



The pebbles are almost all of a roundish oval form, many 

 of them being striped, but differing from those of the su- 

 perior gravel stratum, in being seldom broken, in there be- 

 ing few large ramose masses, and in their not bearing any 

 marks or traces of organization. Many of these pebbles 

 are passing into a slate of decomposition, whence they have 

 in some degree the appearance of having been subjected to 

 the action of fire: small fragments of shells are every where 

 dispersed amongst them. 



Beneath the pebbles is a stratum of light fawn-coloured 

 sand of about ten feet in depth, and immediately under this 

 is the stratum of white sand, which is about five-and-thirty 

 feet deep, and is here seen restino immediately on the chalk. 



At Pkimstcad, about a mile distant in a south-eastern 

 direction, there is a pit, in which the shells, about two 

 years ago, were to be obtained in a much better state of 

 preservation than at New Charlton; but this seam of shells, 

 as the pit has been dug further in, has by degrees become so 

 narrow as to be now nearly lost. In this pit, not only the 

 shells already mentioned were found, but many tolerably 

 perfect specimens of Calyptrcea trochiformis, Lam. Iroclms 

 apertus, Brander. Area; glycemeres, Arcce biot'icce, and 

 many minutes shells in good preservation. All these slicils 

 appear to have entirely lost their animal matter, and not 

 having become imbued with any conncciing impregnation, 

 tliey arc extremely brittle. On examination with a lens, it 

 also appears that in most of ihe specimens nothing of their 

 original surface remains, it having been every where in- 

 dented with impressions of the surrounding minute sand, 



made 



