mid on the Fossil Remains contained in them. 135 



impregnation with silex oF various marine animals of im- 

 known genera, but bearing a close affinity to the alci/07na. 

 These stones display, in general, not only the external form 

 but the internal structure also of these animals. The con^ 

 gregation of many pebbles of this genus, and indeed of the 

 same species, in particular tracts, warrants the conclusion, 

 that these animal substances were thus changed, whilst in- 

 habiting that bottom of a former ocean, which now forms 

 the stratum the contents of which are here sketched. Peb- 

 bles of this description are most frequently found in the 

 gravel pits of Hackney, Islington, Ike. 



Among the traces of organization discoverable in this 

 stratum are casts of echini, which are frequently found 

 among the gravel, and which have generally been supposed 

 to have been washed out of the chalk. But these casts have 

 their origin plainly stamped on them. Their substance is 

 covered with iron; they are almost always of a rude and 

 distorted form, and I apprehend that they are never found 

 with any part of the crust of the animal converted into spar, 

 adherent to them, as is commonly the case with the casts 

 of echini found in chalk. 



A sufficient proof, that these several strata of gravel, sand, 

 Sec. have been deposited by a former ocean, is to be found 

 in a circumstance which does not appear to have been hi- 

 therto sufficiently adverted to. This circumstance is the 

 existence of fossil shells belonging to, and accompanying, 

 the superior part of these strata m particular spots; their 

 absence in other parts being, perhaps, attributable to the re- 

 moval of the upper beds. 



These fossil shells are still found disposed over a very 

 considerable extent. Their nearest situation to the metro- 

 polis is at Walton Nase, a point of land about sixteen miles 

 S. E. of Colchester. Heie a cliff rises more than fifty feel 

 above high-water mark and the adjacent marshes. It is 

 formed of about two feet of vegetable mould, twenty or 

 thirty feet of shells, mixed with sand and gravel, and from 

 ten to fifteen feet of blue clay. The bed of shells is here 

 exposed for about three hundred paces in length, and about 

 a hundred feet in breadth. 



fnuiiediately beyond the Nase the shore suddenly re- 

 cedes and forms a kind of estuary, terminated towards the 

 cast bv the projecting cliff of Harwich, which is capped in 

 a similar manner with beds of these shells. The height of 

 this cliff is from forty to fifty feet, about twenty-two i'eet of 

 the lower part of which is the upper part of the blue clay 

 flralum ; " above which," as Mr. Dale observes, '* to with- 



l4 in 



