482 THE DEPTHS OF THE SEA. [cHap. x. 
work of spaces, vacant, or loosely filled with peroxide 
or carbonate of iron. It therefore seems certain that 
by some means or other the organic silica, distributed 
in the shape of sponge spicules and other silicious 
organisms in the chalk, has been dissolved or reduced 
to a colloid state, and accumulated in moulds formed 
by the shells or outer walls of imbedded animals of 
various classes. How the solution of the silica is 
effected we do not precisely know. Once reduced 
to a colloid condition, it is easy enough to imagine 
that it may be sifted from the water by a process 
of endosmose, the chalk matrix acting as a porous 
medium, and accumulated in any convenient cavities. 
In various localities in the chalk and green-sand of 
the North of England the peculiar bodies which are 
called Ventriculites are excessively abundant,—ele- 
gant vases and cups with branching root-like bases, 
or groups of regularly or irregularly spreading tubes, 
delicately fretted on the surface with an impressed 
network like the finest lace. In the year 1840 the 
late Mr. Toulmin Smith published the result of 
many years’ careful study of these bodies, and gave 
a minute and most accurate description of their 
structure. He found them to consist of tubes of 
extreme tenuity, delicately meshed, and having be- 
tween them interspaces usually with very regular 
cubial or octohedral forms. These tubes in the Ven- 
triculites found in chalk were empty, or contained a 
little red ochreous matter ; but when a ventriculite or 
a portion of one happened to be entangled in a flint, it 
was either incorporated with the flint or replaced by 
silica. Mr. Toulmin Smith supposed that the skeleton 
of the ventriculite had been originally calcareous, 
