Jjg STRATA IN THE NEIGHBOURHOOD OP LONDON. 



chalk, since very considerable variations occur as to their 

 thickness, and indeed as to the form in which their consti- 

 tuent parts are disposed; and since there exist but few 

 sections, at least in the neighbourhood of the metropolis, 

 which present a view of the strata composing this formation. 

 They are included in the following account by Mr. Farey : 

 •* A sand stratum, of very variable thickness, next succeeds, 

 •* and lies immediately upon the chalk, in most instances, 

 ** as between Greenwich and Woolwich, on the banks of 

 •* the Thames; which has often been called the Blackheath 

 ** sand, it frequently has a bed of cherty sandstone in it, 

 ** called the gray-weathers"*. 

 The bottom of On the upper part of a mound at New Charlton some 

 ihe clay traces of the lowest part of the blue clay appear, covered by 



not more than a foot of vegetable earth. This layer of clay 

 does not seem to exceed two feet in thickness, which, indeed, 

 it possesses only on the top of some of those mounds, which 

 occur so frequently as to render the surface in this district 

 contains very irregular. In this clay oysters of different forms are 



found: some approaching to the recent species, and others 

 longer and somewhat vaulted ; but they are in general so 

 tender, as to render it very difficult to obtain a tolerable 

 specimen. With these also occur numerous cerithia, tur* 

 rkellce and cytheretje. Lam. ; all of which are in a similar 

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

 belonging to longing to the subjacent stratum, but whiclj, having lain 

 blnea'th""' "Ppermost, became involved in, the first or lowest depositioa 



of the blue clay. 

 Shells in great Immediately beneath the clay there is found a line of about 

 t'^rter*"'"' ^'>'"^^«'' ^«"»' '"ches of the preceeding shells imbedded in a 

 mass of calcareous matter, the result pf their disintegration. 

 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 fragments of shells are to 

 be found, and in others a calcareous powder only is left. 



• Rejjort on Derbyshire, &c. vol. I, p. HI. 



The 



