and on ihe Fossil Remains contained in them. 151 



their sides^ at the pcriotl at which this iuflUration took 

 place. 



That the separation and deposition of the matter forming 

 these siliceons nodules have been the work ofcrystalhzation, 

 is rendered evident by the cavities left either in thc?e no- 

 dules, or in the fossils, being generally lined with qnartz 

 crystals. 



Whilst endeavouring thus to explain the formation of 

 these fiinty nodules, an"d ihf illling up of the cavities of the 

 fossils with flint, a difiRculty arises from observing these 

 bodies, insulated as it were in their bed of chalk; it not be- 

 ing 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 si- 

 liceous grains. 



Something analogous is however observable in the forma- 

 tion of the calcareous stalactite; since in those caverns iii 

 which these concretions have been forming for a veiy long 

 period, the infiltration by which they are formed is f(HnKl 

 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. 



TheOberstein nodulesof agate appear to have been formed 

 under somewhat similar circumstances ; since it is in ge- 

 neral evident from their externa! surfaces, that they also 

 have had very little adherence to their matrices: which 

 would hardly have been the case had these been highly im- 

 pregnated with siiex. 



The HAKD CHALK lies immediatelv beneath the soft 

 chalk. In this stratum there are no flint nodules. " Its 

 beds," according to Mr. Farcy, " increase in hardness, un- 

 til near the bntiom where a whitish freestone is dug, at 

 Totternhoe in Bedfordshire, and at numerous other places: 

 that brought from Ryegate and other quarries, of this stra- 

 tum, south of London, is used as a fire stone*." 



It has been generally sujiposed that these two strata of 

 chalk are of one formaiion; but not onlv the absence of 

 the flints but the characters of tlicir fossils prove then) In 

 be of tiistincl formations. No fossils indeed are marked 

 by more dicidedly peculiar characters than those of this 

 stratuin ; since hardly a single fossil has been found in it, 

 which has been met witli in the soft chalk, ov any other 

 stratum. 



• Report on Dt:bys.hirc. ^<g. p. 112. 



K4 It 



