Chalk and Flint of the South-east of England. vo 
spongeous tissue, and of spicula, in our flints ; in the presence of 
polythalamia and infusoria, particularly of Xanthidia, in the canals 
of sponges, and their frequent suspension throughout the mass 
of a siliceous nodule ; as if the spongeous tissue had retained its 
form sufficiently long to allow of the silicification of the animal- 
cules, and had subsequently perished. At the same time I must 
express my conviction, that the facts he so faithfully portrays do 
not warrant the hypothesis that all the nodules, veins, dikes, and 
sheets of flint, are to be ascribed to the silicification of sponges ; 
neither can I admit that the cavities of the shells of echinoderms 
and mollusks, now found filled with flint, were previously occu- 
pied by sponges. The theory of M. Ehrenberg, that the compact 
nodules of flint are the consolidated pulverulent siliceous parti- 
cles of infusoria, I conceive to be equally untenable. Nor do the 
facts hitherto brought before us seem to warrant the inference, 
that the abundance of siliceous spicula m any of the porifera 
rendered those bodies more favourable for silicification ; on the 
contrary, the soft gelatinous animal matter, as Mr. Bowerbank 
has suggested, does appear to have exerted such an influence by 
some species of elective affinity or attraction : hence the frequent 
silicification of the bodies of mollusks, while the shell retains its 
calcareous character, as in the specimen of an oyster figured in 
the ‘ Medals of Creation,’ p. 363. 
In many of the silicified fossils of the chalk, the mineralization 
is simply that of incrustation and infiltration ; such is the state of 
numerous sponges, which are, as it were, invested by the flint, and 
haye their pores and tubes filled with the same substance ; but the 
spongeous tissue is in the condition of a brown friable earthy sub- 
stance. In other examples the sponge has been incrusted by a mass 
of liquid silex, and its tissue has subsequently perished ; in this 
manner have been formed those hollow nodules, which, on being 
broken, present a large cavity containing only a little white pow- 
der, or some loose fragments of silicified sponge; while in other 
specimens the cavity is lined with quartz and chalcedony, probably 
introduced by subsequent infiltration through the nodule. It fre- 
quently happens that the zoophyte is only partially invested with 
silex, while the other portion is imbedded in the chalk, and is a 
friable caleareous substance. The Choanites and Ventriculites are 
often found in this condition, and hence the protean forms assumed 
by the flints that have been moulded in the cavities of these orga- 
nisms. These specimens appear to demonstrate that the organic 
bodies became permeated with flint, only when they happened to 
be exposed to the current or stream of liquid silex, which pene- 
trated such portions of structures, or entered the cavities of such 
shells, and echimoderms, as were lying at the bottom of the ocean 
over which it flowed, or were immersed in the calcareous detritus 
