Geological Societ^^ 
503 
ISiK] 
4th. PebbleG, owing their form to an 
investment aurt impregnation with silex 
of various marine animals of unknown 
genera, but bearing a close affinity to 
the alcyonia. These stones display, lii 
general, not only the external form, but 
the internal structure also, of these ani¬ 
mals. The congregation of many pebbles 
of this genus, and indeed of the same 
species, in particular tracts, warrants 
the conclusion, that these animal sub¬ 
stances were thus changed, whilst in¬ 
habiting that bottom of a former ocean, 
which now forms the stratum, the con¬ 
tents of which are here sketched. Peb¬ 
bles of this description are most fre¬ 
quently found in the gravel-pits of Hack¬ 
ney, Islington, &c. 
Among the traces of organization dis¬ 
coverable in this stratum, are casts of 
echini, which are frequently found among 
the gravel- and wdiich 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 circumstance which does not 
appear to have been hitherto sufficiently 
adverted to. This circumstance is tiie 
existence of fossil shells belonging to, 
and accompanying, tire superior part of 
these strata in particular spots; their ab¬ 
sence in other parts being, perhaps, at¬ 
tributable to the removal of tlie upper- 
beds. 
These fossil shells are stijl found dis¬ 
posed over a very considerable extent. 
Their nearest situation to the metro¬ 
polis is at Walton Nase, a point of land 
about sixteen miles south-east of Col¬ 
chester. 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 ft ora ten iso 
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. 
Immediately beyond the Nase the 
shore suddetffy recedes, and forms a 
kmd of estuary^ terminated towaids the 
east by 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 ffeet of the lower part of 
which is the upper part of the blue clay 
stratum: “above which,” as Mr. Dale 
observes, “ to within two feet of the 
surface, are divers strata of sand and 
gravel, mixed with fragments of shells, 
and small pebbles; and it is in some of 
these last-mentioned strata, that the 
fossil shells are imbedded. These fos« 
sils lie promiscuously together, bivalve 
and turbinate, neither do the strata in 
which they lie observe any order, being 
sometimes higher and sometimes lower 
in the cliff; with strata of saiid, gravelj, 
and fragments of shells between. Nor 
do the shells always lie separate or dis¬ 
tinct in the strata, but are sometimes 
found in lumps or masses, something 
friable, cemented together with sand and 
fragments, of a ferruginous or rusty 
colour, of which all these strata are.” 
The coast of Essex is here separated 
from that of Suffolk, by the river Stour, 
by which the continuity of this stratunj 
is necessarily interrupted. It however 
occurs again on the opposite side of the 
river, and through Suffolk and great part 
of Norfolk, the same bed of shells is 
found on digging; thus appearing to ex¬ 
tend over a tract of at least forty miles 
in leiigth. 
These shells are in general found in 
the same confused mixture as is described 
by Mr. Dale; but they are also some¬ 
times so disposed, that patches of par¬ 
ticular geneia and species, appear to be 
now occupying the very spots where they 
had lived. This seems particularly the 
case with the small pec tens, the mactroc^ 
and i\\Q Left-turned whelk 0 ♦ 
From the excellent state of preser¬ 
vation in which many of these shells 
have been found, it has been thought 
that they could hardly be regarded as 
fossil. j\Ianv acknowledged fossil shells, 
however, have undergone much less 
changes than those of this stratum ; the 
original coloured markings are entirely 
discharged, and the external surfaces are 
deeply penetrated with a strong ferru¬ 
ginous stain ; the inner surfaces are al^o 
crrnsiderably clianged, their resplendence 
being superseded, to a considerable 
depth, by a dead whiteness, the con¬ 
sequence of the decomposition of this 
part of the shell. 
J.ike the fossils of most other stratOj 
this assemblage of shells manifests a oe- 
cuiiar 
