Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/342

 indeed of the same species, in particular tracts, warrants the conclusion, 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, Islington, &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, 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, &c. have been deposited by a former ocean, is to be found in a circumstance which does not appear to have been hitherto sufficiently adverted to. This circumstance is the existence of fossil shells belonging to, and accompanying the superior part of these strata in particular 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 more 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 in length, and about a hundred feet in breadth.