Geological Society. 223 



GEOLOGICAL SOCIETY. 



March 11, 181-0. — A paper -was read, "On the Siliceous Bodies of 

 the Chalk, Greensand and Oolites ;" by I\Ir. Rowerbank, F.G.S. 



The author commences by stating, tliat naturalists and geologists 

 have long considered the form of tuberous masses of Hint found in 

 the upper chalk to be due to alcyonia or sponges, but that he is not 

 aware of this opinion having been proved to be correct. It was 

 Professor Ehrenberg's observations on siliceous bodies which first 

 induced him to obtain thin slices of flint with the intention of pro- 

 curing specimens of Xanthidium. In the examination of these slices, 

 he was struck with the frequent occurrence of patches of brown, re- 

 ticulated tissue, spicula and Foraminifera, and he was induced to 

 infer, that the patches of tissue were the remains of the organized 

 body, possibly a sponge, to which the flint owed its form. With this 

 belief, he commenced his inquiries by examining thin slices of flints 

 obtained from various localities, and he found in all of them a per- 

 fect accordance in the structure and proportion of reticulated tissue, 

 in the number of spicula, and in the occurrence of Xanthidia and 

 Foraminifera. The following are the general appearances which the 

 slices of flint exliibit when mounted upon glass. 



With a power of about 120 linear, the slice presents the appear- 

 ance of a stratum of a turbid solution of decomposed vegetable or 

 animal matter containing Foraminifera, spicula, Xanthidia, and fre- 

 quently fragments of the brow n tissue. In a specimen from North- 

 fleet the mass of the spongeous portion exhibited numerous cylin- 

 drical contorted canals, which from their uniformity and minuteness 

 of diameter, Mr. Bowerbank considered to be the incurrent canals of 

 the sponge ; and other orifices of greater diameter to be the excur- 

 rent. Very frequently, when little of the reticulated substance of the 

 sponge remains, its former presence, the author says, is indicated by the 

 siliceous matter resembling a congeries of gelatinous globules, mould- 

 ed by the tissue amid which it was deposited ; and the globules, when 

 traced to the edges of the patches of spongeous texture, were found 

 to agree in size and form with the orifices of the supposed incurrent 

 canals. In cases where no traces of the sponge can be detected, Mr. 

 Bowerbank thinks, that the mode in which the spicula, Foraminifera 

 and other extraneous matters are dispersed equally in all parts, and 

 not precipitated to one portion of the flint, indicates that the organ- 

 ized tissue in which they were entangled, retained its form and tex- 

 ture sufficiently long to allow of the fossilization of these remains in 

 their original places ; and that the nature and position of these bodies 

 strongly indicated the former spongeous nature of the flint. 



When the chalk is carefully washed from the exterior of a flint, 

 and a portion examined as an opake object with a power of about 

 fiftj^ linear, it exhibits a peculiar saccharine appearance, with deep 

 circular excavations, having fragments of extraneous matters partly 

 imbedded or adhering to them. If the surface be further cleansed by 

 immersion in diluted muriatic acid, till effervescence ceases, spicula 

 may be detected on the sides of the deep circular cavities ; and if, 



