Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/439

 been subjected to the solvent action of water under some particular modification, by which nearly the whole interstitial matter (with the exception of a few flakes here and there of quartzy chalcedony) has been removed, while the quartz moulded within the tubes of madrepore and representing most perfectly the external form of the zoophyte, alone remains.

The only difference that chemical analysis has detected between quartz and chalcedony, is that the former is silex with perhaps one or two per cent. of water, while the latter contains, besides, about two per cent. of alumine and lime; but this difference appears by no means sufficient to account for the absolute permanence of the one, and the readiness with which the other suffers decomposition under the same circumstances. The moisture contained in the bed is primarily rain water, and it is not easy to see what active agent it can become charged with in draining into the interior of the mass, except carbonic acid or carbonate of lime: any of the stronger acids, such as the sulphuric, resulting from the decomposition of pyrites, would immediately be neutralized by the calcareous matter in which the whole bed abounds. Alternations of moisture and dryness, of heat and cold, to which the decomposition of the exposed surfaces of rocks is so generally attributed, could have little or no influence in this case; since the state of moisture and of temperature at the depth of twenty feet or more in a bed of gravel cannot be supposed to undergo much change.

May I take the liberty of recommending to the well known activity of the members of this Society an examination into the circumstances on which the phenomena mentioned above depend, and which appear to be not a little important both in a chemical and geological point of view?