216 Mr. W. Murton Holmes on 
Barbadoes and many other localities, and many paleozoic rocks 
have now been proved to owe their origin to radiolaria. Our 
fellow-member, Dr. Hinde, has discovered their existence in 
many rocks not previously believed to be of organic origin, and he 
frequently receives fresh specimens from all parts of the globe. 
Sponges, again, are silica producers. With the exception of 
two groups all the members of this large family have siliceous 
spicules in greater or less abundance. Some of these spicules 
are of comparatively large size, while others are very minute. 
These spicules are found in some parts of the ocean-bed in large 
quantities, and their fossil remains form a considerable part of 
the greensand formation. In the recent condition, the larger 
spicules very much resemble spun glass. They are flexible, and 
possess considerable elasticity. 
The silica in all these organisms, when comparatively recent, 
is in the hydrous or colloid condition, chemically resembling 
opal. Examined under the microscope, they give no reaction 
with polarized light. In the older fossil deposits a change has 
taken place, and the organisms, though outwardly retaining 
their original form, have undergone a physical change. They 
have lost water and become crystalline, so that they affect a ray 
of polarized light. 
This organic silica is frequently dissolved and re-deposited in 
another form. The sea-bottom forming what has been called 
‘¢ modern chalk ’’ contains, in addition to numerous calcareous 
organisms, a certain amount of silica diffused through it. In 
the true chalk all the silica is found collected into layers or 
nodular masses of flint. The question is, how are these formed ? 
I think in all probability some organisms, such as an echinus, a 
sponge, or a mollusc, when in a state of incipient decomposition, 
set free a small quantity of silica from solution, which was 
deposited, and so, particle by particle, the original calcareous 
shell was replaced by one of silica; or the mass of siliceous 
spicules of the sponge became gradually embedded in a mass of 
jelly-like silica,—for when once a mass of silica had become 
‘isolated, the tendency would be for more to collect round it, just _ 
as crystals of alum will grow from a solution, until the greater 
part of the silica in the surrounding ooze had been collected 
together. That the silica must have been in a gelatinous con- 
dition at one time is certain, for we often find hollow flints 
enclosing portions of the sea-bottom. It is in these cavities 
that one finds the smaller kinds of chalk fossils best preserved. 
In the course of time the colloid silica would be altered into the 
semi-crystalline condition of flint by giving up some of its water 
of combination. Flints differ from other varieties of silica, 
inasmuch as they break with a conchoidal or shell-like fracture, 
and have sharp cutting edges, 
