Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/363

 infiltration, with flint.” The decomposition of the softer parts of the animals, which were thus entombed, may be considered as a very probable source of a part of those gaseous matters which formed these cavities: and the connection of the animal remains with these nodules of flint is easily explained by supposing the shells, crusts of the echini, &c. to have projected into these cavities, or to have been adherent to their sides, at the period at which this infiltration took place.

That the separation and deposition of the matter forming these siliceous nodules have been the work of crystallization, is rendered evident by the cavities left either in these nodules, or in the fossils, being generally lined with quartz crystals.

Whilst endeavouring thus to explain the formation of these flinty nodules, and the filling up of the cavities of the fossils with flint, a difficulty arises from observing these bodies, insulated as it were in their bed of chalk: it not being easy to conceive, how so copious an infiltration should have taken place into these cavities, whilst the surrounding chalk should only have received a slight intermixture of siliceous grains.

Something analogous is however observable in the formation of the calcareous stalactite; since in those cavern in which these. concretions have been forming for a very long period, the infiltration by which they are formed is found to continue to the present day; proving that the interstices of the superincumbent stone, have not yet been filled by the concreting of the earthy particles held in solution in the percolating fluid, by the crystallization of which these bodies have been formed, and are now augmenting.

The Oberstein nodules of agate appear to have been formed under somewhat similar circumstances; since it is in general evident from their external surfaces, that they also have had very little adherence to