398 



CRETACEOUS. 



that have served as niicki and to some extent moulded their outlines. Of these 

 nuclei Sponges are most abundant, but they also include Mollusca, Echinoderms, 

 Reptilian teeth, and other organic remains met with in the Chalk. That the flints 

 have been formed subsequently to the deposition of the Chalk that immediately 

 surrounds them, is indicated by their shape, which could not have been assumed 

 unless the siliceous matter was enveloped and supported, and by the vertical lines 

 of paramoudras which are sometimes connected. That some varieties of flint were 

 formed subsequently to the consolidation of the beds, is suggested by the occasional 

 occurrence of seams of " tabular flint," which may occupy horizontal planes, or fill 

 vertical joints.^ Thin seams of flint running in different directions have been noted 

 in the Chalk at Wells in Norfolk, at Worthing, Rottingdean, and other places. 

 In most cases the accumulations of flint have been formed before the disturbances 

 to which Chalk has in many places been subjected. The reconstruction, however, 

 of inorganic silica from mineral solutions, as suggested in 1S52 by M. Gaudry, 

 may account for some of the local veins of flint. The following is an analysis of 

 Chalk-flint by Klaproth : - — 



Silica qS'OO 



Lime 0-50 



Alumina o'25 



Oxide of Iron 0*25 



Water i "oo 



The silica itself was derived in the first instance from the sea-water (which 

 contains a minute per-centage of it), partly by organisms which have to provide 

 themselves with a siliceous framework, as certain Sponges, Polycistina, and 

 Diatomacese. The decay of these, and especially of Sponges, would yield most of the 

 material for the flints in the Chalk ; and further, it is considered that the decom- 

 position of certain organisms would produce chemical changes sufficient to eliminate 

 silica from the sea-water, and to assist in its deposition. 



From the fact that the majority of flints exhibit spongiform structure, it has been 

 argued that their method of formation was essentially organic. It may, however, 

 be said that the majority of flints enclose organisms, such as Foraminifera, that 

 were once calcareous, and hence it seems clear that while sponges furnished a great 

 part of the silica, they also acted as nuclei, around which siliceous matter was 

 deposited. Sir C. Wyville Thomson observed that we often find the moulds and 

 outlines of organisms considered to have been siliceous, from which the whole 

 of the silica has been removed ; and cases occur in which a portion of the delicate 

 tracery of a siliceous sponge has been preserved entire in a flint, while the 

 remainder of the vase which projected beyond the outline of the flint appeared 

 in the Chalk as a trellis-work of spaces, vacant or loosely filled with peroxide or 

 carbonate of iron. On the other hand, calcareous organisms have been perfectly 

 replaced by silica in the white powder within some flints, as remarked by Prof. T. 

 Rupert Jones and Mr. Joseph Wright. 



The most puzzling point is the origin of the layers of flint. Their comparative 

 regularity, looked at in a large way, points to some connection with the stratifica- 

 tion of the rock. In this respect they are analogous to the septaria and clay-iron- 

 stone nodules of many formations, and to certain forms of alabaster met with 

 in the Keuper Marl. The remarkable concentric rings of flint to be seen on the 

 foreshore at Runton, near Cromer, are worthy of mention. There a platform of 

 Chalk is exposed which exhibits rings, some made up of flints connected, others 

 disconnected, and they often surround hard Chalk with isolated flints or a Para- 

 moudra of gigantic dimensions ; here they seem to be due to a kind of mineral 



^ Geology of Norwich (Geol. Survey), p. 20. See also G. J. Hinde, Fossil 

 Sponge Spicules from the Upper Chalk (Horstead), 1880 ; W. J. Solias, Ann. Nat. 

 Hist. 18S0, pp. 384, 437. 



^ Dixon, Geol. Sussex, edit. 2, p. 125. 



d 



