166 
THE GEOLOGIST. 
quently their attached shells are found uncovered in somewhat deep 
depressions, or rather hollows in the flint nodule. 
The attachment of some very young oysters on the raised portions 
of the piece of flint nodule we have figured at a later period, either 
on the enveloping sponge after its life was arrested, or on the 
hardened surface of the nodule itself, is an excuse or a reason (which- 
ever the reader pleases) for a slight digression on the formation of 
flint, and which we should not make, did w^e not desire to figure a 
very instructive specimen from the collection of Dr. Bowerbank 
(Plate X.). It consists of the shell of a galerite imbedded in flint, 
a very common occurrence, though not commonly met with in such 
an illustrative manner. 
The shell evidently must have been siipported, for the flint ex- 
tends far beneath it. It could not have rested three or four inches 
above the cretaceous mud without something tolerably solid under- 
neath to uphold it. A sponge would do this eftectually; silex in 
a dense gelatinous state would also support it, but th-en the gela- 
tinous substance would yield to pressure, and the shell, or whatever 
it was which rested on it, would incline to one side or other, accord- 
ing to its natural angle of rest. In the example figured, a sponge 
has grown all round, and covered with a plastering or film the whole 
of the interior of the galerite, while the life-existence of it was cut 
ofl" or its grow^th arrested for want of a proper circulation of water, 
when the more dense mass of sponge w-hich grew up from the floor 
of the galerite attained two-thirds the height of the cavity. The 
growth of the supporting sponge went on outside the galerite in a 
flourishing condition, and the shell was at last wholly and thickly 
enveloped. 
Then the hardening into flint went on, and after the nodule had 
been formed the calcareous shell-matter of the galerite was slowly 
dissolved out — for water percolates even flints — and a cavity was 
formed between the thin inner film of sponge or flint and the flint 
cast of the exterior of the shell. This thin inner film is most valu- 
able evidence here, for we could not account for it by the gelatinous 
deposition of silex, while we can explain it by the growth of sponge. 
It might, it is true, be formed by the oozing through the shell of the 
galerite of water containing minute quantities of flint in solution. 
But then its evenness over the roof and sides alike would be a little 
strange. 
From the consolidation of flint around the lower parts of the 
stems and roots of ventriculites, much of their former living nature 
