S30 Mr. Parkinson o« /^^ Strata^ 



Indeed of the same species, in particular tracts, warrants the con- 

 clusion, that these animal substances were thus changed, whilst 

 inhabiting that bottom of a former ocean, which now forms the 

 stratum the contents of which are here sketched. Pebbles of this 

 description are most frequently found in the gravel pits of Hackney, 

 IsUngton, &c. 



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, ad- 

 herent 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, &;c. 

 have been deposited by a former ocean, is to be found in a circum- 

 stance which does not appear to have been hitherto sufficiently ad- 

 verted to. This circumstance is the existence of fossil shells belong- 

 ing to, and accompanying the superior part of these strata in particu- 

 lar spots : their absence in other parts being, perhaps, attributable to 

 the removal of the upper beds. 



These fossil shells are still found disposed over a very considerable 

 extent. Their nearest situation to the metropolis is at Walton Nase^ 

 a point of land about sixteen miles S. E. of Colchester. Here a clifF 

 rises mora than fifty feet 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 ia length, aiid about a hundred feet hi breadth. 



