34. PRINCIPLES OF PALAONTOLOGY. 
known as chert, are found in limestones of almost all ages from 
the Silurian upwards; but they are especially abundant in the 
chalk. When these flints are examined in thin and trans- 
parent slices under the microscope, or in polished sections, 
they are found to contain an abundance of minute organic 
bodies—such as Foraminifera, sponge-spicules, &c.—embedded 
in a siliceous basis. In many instances the flint contains 
larger organisms—such as a Sponge or a Sea-urchin: As the 
flint has completely surrounded and infiltrated the fossils which 
it contains, it is obvious that it must have been deposited from 
sea-water in a gelatinous condition, and subsequently have 
hardened. ‘That silica is capable of assuming this viscous and 
soluble condition is known; and the formation of flint may 
therefore be regarded as due to the separation of silica from 
the sea-water and its deposition round some organic body in a 
state of chemical change or decay, just as nodules of phos- 
phate of lime or carbonate of iron are produced. ‘The exist- 
ence of numerous organic bodies in flint has long been known; 
but it should be added that a recent observer (Mr Hawkins 
Johnson) asserts that the existence of an organic structure can 
be demonstrated by suitable methods of treatment, even in the 
actual matrix or basis of the flint.* 
In addition to deposits formed of flint itself, there are other 
siliceous deposits formed by certain szccates, and also of 
organic origin. It has been shown, namely—by observations 
carried out in our present seas—that the shells of Aoraminzfera 
are liable to become completely infiltrated by silicates (such 
as ‘‘ glauconite,” or silicate of iron and potash). Should the 
actual calcareous shell become dissolved away subsequent to 
this infiltration—as is also liable to occur—then, in place of 
the shells of the Foraminifera, we get a corresponding number 
of green sandy grains of glauconite, each grain being the cast 
of a single shell. It has thus been shown that the green sand 
found covering the sea-bottom in certain localities (as found 
by the Challenger expedition along the line of the Agulhas 
current) is really organic, and is composed of casts of the 
shells of Foraminifera. Long before these observations had 
been made, it had been shown by Professor Ehrenberg that 
the green sands of various geological formations are composed 
mainly of the internal casts of the shells of Foraminifera ; and 
* It has been asserted that the flints of the chalk are merely fossil 
sponges. No explanation of the origin of flint, however, can be satisfac- 
tory, unless it embraces the origin of chert in almost all great limestones 
from the Silurian upwards, as well as the common phenomenon of the 
silicification of organic bodies (such as corals and shells) which are known 
with certainty to have been originally calcareous. 
