Formation of Flints in Chalk. 417 



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 sili- 

 ceous 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 occu- 

 pied, 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 case in 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, con- 

 tinuous tabular masses.* 



* It happens occasionally that very narrow fissures, traversing the chalk and cutting 

 two or three of its siliceous strata, arc filled with tabular plates of black flint. Such 



