STRATA IN THE NEIGHBOURHOOD OF LONDON. ^.g ' 



stance, which is seldom bhick, but exhibits hhadea of 

 yellow or brown, in wiiich red likewise is sometimes per- 

 <:eptible. The traces of organic structure, particuiarl}- of Organic traces 

 the alcyonmm, occasionally seen in these stones, determine*" ^^^' 

 them also to have been formed at the bottom of the sea. 



4th. Pebbles, owing their form to an investment and 4th kind. Ma- 

 impregnation with silex, of various marine animals of un- -nj^^recimed 

 known genera, but bearing a close affinity to the o/ryow/a. or invest«d 

 These stones display, in general, not only the external form, ^'^^ ^'*^*-» 

 but the internal structure also of these animuls. 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, while in- while at t^e 

 habiting tlie bottom of a former ocean, which now forms j^^°'" '^ 

 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, &:c. 



Among the traces of organization discoverable in this Casts of 

 stratum are casts o( echini, which are frequently found ^^^'^'» 

 among the gravel, and which have generally been supposed not 'A-aslied 

 to have been washed out of the chalk. But these casts *'" o c, a j£„ 

 have their origin plainly st:im|)e<l on them. Their substance 

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

 and distorted form ; and 1 apprehend, that they are never 

 found with any part of the crjust of the animal convertfd 

 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, dossil shells 

 sand, &c. have been deposited by a former ocean, is to be accompanying 

 found in a circumstance, which does not appear to have ^ ^^*^ strata, 

 been hitherto sufficiently adverted to. This circumstance 

 is the existence of fossil shells belonging to, and accompa- 

 nying, the superior part of these strata in particular spots: 

 their absence in other parts bring, perhaps, attributable to 

 the reraoval'of the upper beds. 



These fossil shells are still found disposed over a very <l'sposed over 

 considerable extent. Their nearest situation to the metro- ^ considerable 



. . extern now. 



polls 13 at Walton Nase, a point of land about sixteen miles At Walton 



S. E. of Colchester. Here a cliff rises more than fifty feet ^'^'''' 



above high water mark and the adjacent raarsihe?. It is 



formed 



