432 
DR. G. J. HINDE ON BEDS OF SPONGE-REMAINS IN THE 
the colloid condition the sponge-beds are usually friable and incoherent, but where it 
has been redeposited as chalcedony, the beds are of a hard, resistant character, and 
serve, as already mentioned, for whetstones and other economical purposes. 
Beds of chert. —The chert associated with the sponge-beds of the lower and upper 
greensand varies from a light pellucid to a dark brown tint; it is readily translucent 
in thin sections. It is highly brittle, and sufficiently hard to scratch glass. It shows 
no reaction in nitric acid, and may thus be assumed to be free from calcite. Thin 
microscopic sections exhibit a nearly transparent ground-mass of chalcedonic—some¬ 
times partially also of crystalline—silica, in which are numerous sponge-spicules, and 
also occasionally entire and fragmentary foraminifera and fragments of echinoderm 
structure. In some cases also grains of quartz-sand and glauconite are present. The 
chalcedonic silica forming the ground-mass can be seen in many cases to have been 
deposited in concentric layers over the enclosed siliceous and calcareous organic 
remains; it also, when partially crystalline, exhibits a radiate fibrous structure 
(Plate 40, fig. 3). 
Throughout the chert, sponge-spicules are present, but owing to the fact that the 
spicules are usually of chalcedony, like the ground-mass of the chert in which they 
are enclosed, their outlines are very indistinct. But even when the spicules them¬ 
selves cannot for this reason be recognised, their former presence is indicated by the 
solidified canals, which being of glauconite, or of silica of a different tint to that of 
the matrix, are distinctly shown (Plate 40, fig. 3.). In some cases the spicules have 
been dissolved after the chert has formed round them, leaving empty moulds. 
Intimately allied to the chert is the yellowish-gray porous rock which frequently 
forms an exterior crust to the chert, but sometimes also forms independent beds. 
The rock is principally of chalcedonic silica, with which very frequently calcite is 
intermingled. The main difference between this rock and chert consists in its porous 
nature, arising from the fact that the spicules in it are but as empty casts, and the 
silica of these spicules has probably been deposited in the adjoining bands of chert. 
There can scarcely be room for doubting that the beds and irregular masses of chert, 
which are found nearly everywhere in the strata of the lower and upper greensand in 
connexion with the detached spicules of sponges, have been derived from the silica of 
these sponge-remains ; and from the same source has also originated the silica which, 
in many of the deposits, more particularly in the Blackdown Hills, has replaced the 
shells and tests of the mollusca and other calcareous organisms. The theory has, 
however, been advocated that the silica of chert has been derived rather as a direct 
deposit of this mineral from solution in sea-water, than as the product of the decom¬ 
position of the siliceous structure of sponges. Thus Dr. Bowerbank * held that the 
sponges imbedded in the chert of the greensand possessed horny and not siliceous 
skeletons, and that the silica of the chert in which they were imbedded was attracted 
from the exterior medium by the animal matter, and not secreted therefrom by the 
* Trans. Geol. Soc., ser. 2, vol. vi., p. 181. 
