— 8o — 



chalk itself, which has there been metamorphosed into a hard 

 limestone, no traces of sponges remain, and nearly all recog-, 

 nizable fragments even of the calcareous organisms have been 

 obliterated. It is possible that a small portion of the silica 

 of the flints may be derived from other organisms such as 

 the Radiolaria ; but up to the present the only two or three 

 forms of this group known to exist in the Chalk formation 

 have been discovered by Prof Zittel in strata in Westphalia 

 teeming with unaltered sponge spicules ; and so far as known 

 at present the Radiolarians had but a slight de\'elopment in 

 the cretaceous ocean. 



The hypothesis of the derivation of the flints in the chalk 

 from the silica of sponge skeletons is by no means a ne«' 

 one, but it has not hitherto been generally accepted from the 

 ab.sence of proof of the existence of sponges in sufficient 

 numbers to furnish silica for the great mass of flints A\'ith 

 which the chalk is filled, and from the difficulty of explaining 

 the arrangement of the flints in nodular layers. Even in the 

 last edition of the Students Elements of Geology published 

 in 1 87 1, Sir Charles Lyell attributed the chalk flints to the 

 dissolved shells of diatoms which at certain periods swarmed 

 in the cretaceous ocean and were alternately replaced by the 

 foramenifera which supplied the material for the chalk. Within 

 this present j'ear the origin of the cretaceous flints has been 

 made the subject of a paper by Dr. Wallich. (Quart. Jour. 

 Geo. Soc. 1880, p. 68). This able in\'estigator brought no 

 evidence to sho\\' the general distribution of the sponges in 

 the cretaceous ocean, but merel)' based his argument that the 

 flints were, derived from sponge skeletons, on the fact, that 

 siliceous sponges are found abundantly in the deep sea dredg- 

 ings of the Atlantic, and that on the assumption of their 

 being present in similar numbers in the Chalk ocean, they 

 would have been able to furni.sh the silica for the flints. The 

 contents of this flint from Horstcad and of those from the 

 North of Ireland, prove, what Dr. Wallich assumed, that in 



