Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/430



It is probable that in cases where the body perished rapidly, there was not time for this process of gradual substitution, and that flinty matter of nearly the same coarse quality with the outer crust, was introduced hastily into the void spaces that were left unoccupied by the rapid decay of the animal nucleus. This coarser process is that which appears to have taken place in a vast plurality of instances, amongst which we must reckon that of the Irish Paramoudra, whose history has led us to the present discussion on chalk flints, among which they attain a size unusually gigantic, and often a weight of nearly two hundred pounds.

With respect to the general history of flinty nodules in chalk, whether insulated irregularly or disposed at certain distances in horizontal lines, I must observe that they seem to have originated from causes not dissimilar to those which have produced both nodules and horizontal beds of chert in the calcareous strata of many other secondary formations; e.g. in the freestone of Portland, in the mountain lime of Mells in the Mendip hills, and in the oolitic limestone near Pickering in Yorkshire, and near Poligny, on the north-west edge of the Jura mountains; at which latter place are extensive strata of chert disposed altogether in small nodules resembling chalk flints, as to their shape, size, and position, and without any organic nuclei. The chief difference appears to be, that in the case of the chalk formation, the nodular arrangement of the siliceous strata very much predominates, and in the other cases the siliceous strata, though occasionally nodular (as at Poligny), yet most frequently are disposed in continuous or nearly continuous flat masses; though even these sometimes pass into imperfectly lenticular and tuberous concretions. The existence of insulated siliceous concretions irregularly disseminated through limestone, is common to almost all calcareous strata in which there is any admixture