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some large enough to be seen with the naked eye, fall out 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 xanthidia — 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, molluscs, &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 irom the waters for 

 the formation of a skeleton, say like that of the Euplectella, 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 carbon, 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 



