Mr. E. T. Harpman—On the Chemical Composition of Chert. 93 
of dead organisms. This would give rise to a variety of soluble matter, which 
would be taken up by the sea water; and here again insoluble silica would 
be deposited. And another point must be noted here, not only pseudomorphism, 
but something of the nature of isomorphism may come into play. It is well 
known that the compounds of carbon and of silicium present great similarities, 
Everyone is acquainted with the fact that both have three distinct modifications, 
the crystalline, the graphitoidal, and the amorphous; but not only this: it has 
been shown that the silicium compounds are built up exactly like the carbon ones, 
and that the chemistry of silicium is similar in every respect to what is known as 
organic chemistry, or “the chemistry of Carbon.” Nothing is more likely there- 
fore, than that the relation between them should help to determine the substitution 
of the silicic anhydride for the carbonic anhydride given off during organic 
decomposition. 
That silica readily replaces organic matter is well proven. It is only necessary 
to refer to the well known wood opals—the silicified woods which are met with in 
the Tertiary deposits of many places in the world. In New Zealand; the Rocky 
Mountains ; and at home in the neighbourhood of Lough Neagh, Ireland. In these 
specimens the organic tissue has been replaced bit by bit by silica.* It might also 
be inferred that it rather exerts an elective affinity in favour of organic matter, for 
in many limestones the corals and more prominent fossils are almost completely 
silicified, while the limestone paste has been but little affected. I have lately examined 
many of the limestones in the neighbourhood of Kilkenny, especially that of the 
marble quarries, which is extremely fossiliferous, and I find the fossils to a greater 
or less extent silicified, whilst the limestone is comparatively unaffected. ‘Thus it 
will be seen that the silica could replace both the inorganic and organic matter of 
such a marine deposit as limestone. 
Although I throw out the above suggestion with regard to the result of molecular 
affinity, the point as to difference of solubility is perhaps more vital to our question, 
for most of the silicification is probably due to this cause. As an illustration, [ may 
mention, that calcite often replaces organic matter for this reason. Daubrée? has 
described the petrifaction, in part, of the wooden piles sunk by the Romans in the 
foundations of structures at Bourbonne-les-Bains, the petrifying material being 
ealcite. Daubrée remarks that no calcareous incrustations are found in proximity 
to the mineralized wood, so that the vegetable tissue must by a kind of selection 
have drawn the calcium carbonate to itself and concentrated it in its cells. The 
likelihood is that the water charged with calcium carbonate met with nothing more 
soluble in the surrounding soil, and therefore, passed through it without depositing 
any calcareous matter, but those portions of water which came in contact with the 
decaying organic matter yielded up their carbonate of lime and removed the more 
* “The process of petrifaction of organic bodies is in reality a species of pseudomorphic formation.” 
Elem. of Mineral., J. Nicol, r.r.s.u., &c. (1873), p. 53. See also Dana’s Manual of Min. (1867), p. 54. 
+ Mineralization of organic remains. A. Daubrée, Compt. Rend. Ixxxi., 1008-1010. 
