16 
some large enough to be seen with the naked eye, fallout loosely 
from the interior. There can be no doubt of the sponge origin of 
such flints. Then, though we regard flint as one of the hardest 
substances-we have, look at those specimens before you in which 
you get a perfect model of a whole shell, or in which. you find a 
fossil shell, itself silicified, partly in, and partly out of the solid 
flint. It did not get in there after the flint was formed, it was 
there first, and the silica must have settled round it. And even 
the darkest, densest parts of flint show in microscopic sections 
numerous organisms, notably wanthidia—round minute bodies 
studded with rays, which are believed to be gemmules, the imma- 
ture forms of sponges. What does all this point to? Let us see 
what light has been thrown upon flint formation by those who 
have carefully examined and studied the subject. ‘‘ Constant 
association of flints with traces more or less marked of former 
abundant siliceous organisms seems to make the inference irresist- 
ible that the substance of the flint has been derived from these 
organisms. The silica has first been abstracted from sea water by 
living organisms. It has then been re-dissolved and re-deposited 
(probably through the agency of decomposing organic matter), 
sometimes in amorphous concretions, sometimes replacing the 
calcareous parts of echini, molluses,&c., while the surrounding matrix 
was doubtless still a soft watery ooze under the sea.” (Geikie.) 
Taking this compressed account, let me endeavour to explain 
and illustrate it. We have already seen that certain creatures in 
the sea have the power of secreting silica from the waters to form 
some kind of framework or protection for their soft, gelatinous 
bodies. No doubt the case of the coral polyp occurs to you, 
extracting carbonate of lime from the waters, and we might too 
hastily jump to the conclusion that flint was formed in a similar 
way to coral. But it can hardly be so. Whatever power the crea- 
ture may possess during life to extract silica trom the waters for 
the formation of a skeleton, say like that of the Huplectella, for a 
chastely ornamented shell like that of a diatom, or for spicules like 
those found in some sponges, it would not wholly envelope itself 
in the substance, since that must needs hasten its own destruction. 
The flint was formed after the death of the creature whose remains 
are enclosed in it. In fact its deposition was a chemical operation. 
One of the ultimate elements of organic matter, vegetable or 
animal, is éarbon, and:the bodies of all creatures give off after 
death a quantity of carbonic acid. Now there is a very close 
chemical relation between carbon and silicon, the element out of 
which silica is formed. ; Both come into the same natural group of 
elements, both form similar series of compounds with other 
elements. And we generally find in such cases that. to a certain 
