508 
nunute extraneous bodies which are frequent in the tubular spongi- 
form crust. The character of these fossil sponges differs from that 
of any recent sponge. 
In chert from the greensand of Fovant, Wilts, and from Lyme 
Regis, Mr. Bowerbank found a similar but coarser texture; and 
also in chert casts of Spatangi from the greensand near Shaftes- 
bury. In chert from Portland and Tisbury he found similar cellular 
tissue, but larger, and in texture more like the modern freshwater 
sponge. 
Mr. Bowerbank supposes the organic matter of the sponges and 
zoophytes to have afforded to the silex stronger centres of attrac- 
tion than were offered by the siliceous spicula of the sponges ; and 
there is a geological consideration which seems to favour the hy- 
pothesis, of the siliceous matter of chalk flints whilst in a semifluid 
state having been segregated from the compound mass of lime and 
silex of the nascent chalk beds, by the attraction of some organic 
body, in the facts that the upper region of the English soft chalk, 
which most abounds in flints, is nearly pure carbonate of lime; whilst 
the lower region of the hard chalk is usually destitute of flints, and 
‘has silex diffused throughout its entire substance. I cannot, how- 
ever, but think there is something too exclusive in Mr. Bowerbank’s 
theory as to the universal presence of parasitic sponges in the ex- 
ternal crust of every chalk-flint, and which admits of no case in 
which an Alcyonium or any kind of extraneous body in chalk may, 
without the co-operation of a sponge, have become externally in- 
vested with a crust of silex of the same kind with that which he 
allows to have been attracted to corallines and aleyonic bodies by 
the animal matter they contained. 
MICROSCOPIC SHELLS. 
Mr, Tennant has informed me that a microscopic examination of 
the Stonesfield slate by Mr. Darker, and of other oolites, has re- 
cently shown them to be crowded with remains of organized bodies, 
invisible to the naked eye. I learn also from Mr. Tennant that 
abundant microscopic organic remains have recently been discovered 
_in thin slices of certain beds of carboniferous limestone from Derby- 
shire; similar results may shortly be expected from a microscopic 
examination of the chert of the same formation. We must not 
however be tempted by these discoveries to rush suddenly to the 
rash and unwarranted conclusion, that all limestone and all silex is 
of organic origin. 
It has not yet been shown that the granules resembling the roe of 
fishes, which give character to the oolite formation, and abound oc- 
easionally in limestones of the triassic, carboniferous, and silurian 
series, have any necessary connexion with organic bodies. We may - 
with Ehrenberg admit and admire the extent of microscopic cham- 
bered shells and Infusoria, which he has shown so largely to pervade 
the chalk and other calcareous and siliceous formations, without 
claiming an exclusively animal origin for the entire substance of all 
rocks in which lime or silex are the principal ingredients. “ 
