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XVII. — On the Siliceous Bodies of the Chalk, Greensands, and Oolites. 

 By J. S. BOWERBANK, Esq., F.G.S. 



[Read March 11th, 1840.] 



Plates XVIIJ. and XIX. 



T. HE singular forms presented by the tuberous masses of flint found in the upper 

 chalk have long induced naturaUsts and geologists to imagine that their forms 

 were derived from Alcyonic or spongeous bodies, but I am not aware that this 

 has hitherto been demonstrated to be the actual fact. Professor Ehrenberg's 

 observations on siliceous bodies first induced me, in common with many other 

 persons, to cause thin slices of flints to be made, with the intention of procuring 

 specimens of Xanthidia ; and in ^the examination of these sUces I was struck 

 with the frequent occurrence of small patches of a brown reticulated tissue, which 

 always presented nearly the same appearance. The occurrence of this tissue, 

 combined with the circumstance of finding spicula exhibiting nearly the same form 

 and size, and always occurring in about the same proportion, along with numerous 

 foraminated shells and other extraneous bodies, strongly induced me to believe 

 that the brown reticulated tissue was a portion of the remains of the organized 

 body, the shape of which was represented by the flint nodules ; and the indications 

 thus observed equally inclined me to believe, that if these flints were fossil organ- 

 ized bodies, they would almost inevitably prove to be sponges. With these 

 views I commenced the investigation of the flints of the chalk, and as a prehmi- 

 nary measure I first examined, in a cursory manner, thin slices of flint nodules 

 from various localities, especially of those from the neighbourhoods of Gravesend 

 and Brighton, and from Hertfordshire, Norfolk, and Wiltshire ; and in all of them I 

 found a perfect accordance in the structure and proportion of the reticulated tissue, 

 and of the spicula before alluded to, and also in the occurrence of Xanthidia and 

 ForaminifercB. This similarity in the structure of the flints from all the above local- 

 ities, renders it unnecessary to detail the results of each examination separately. 

 I shall therefore confine myself to the description of what may be observed, by a 



