Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/427

 nodules of chert in other limestone rocks. The present shape of many chalk flints being that of organic bodies, demonstrates the latter to have existed before the consolidation of the former; for the fidelity with which the silex has often copied the organization, and even the accidents and irregularities of the bodies enveloped, is so accurate, that it is impossible to attribute the form of the flint to any other cause than that of the body on which it was deposited. Sometimes the organization is so delicately retained, that it seems not to have undergone the smallest derangement before the siliceous cast was taken; and the model is thus permanently preserved. In other cases the minute fibres and tubes of the animal are not expressed by the silex which has filled the spaces which they occupied, yet the external form represents with faithful accuracy that of the body which afforded to the silex its mould or nucleus. This appears to have been the casein a remarkable degree in the instance of the Irish Paramoudra.

Before the consolidation of the original compound fluid which is now hardened and separated into beds and nodules of flint and chalk, a variety of organic bodies being dispersed through its mass would afford a number of nuclei, to which, in separating itself from the chalk, the silex seems to have had a tendency to attach itself Hence the insulated nodules that occur irregularly in the chalk, out of the line of the flinty strata, do I believe very frequently bear traces of an organic nucleus; so also in many cases do those that occupy the flinty strata. But the greater number of these latter, though their form be usually that of nodules separated from each other by an intervening portion of chalk, yet indicate no traces that refer them to organic origin, and are sometimes extended into thin, continuous tabular masses.