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XXV. ON THE SILICIOUS BODIES OF THE CHALK, GREENSANDS, 

 AND OOLITES. 



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

 [Continued from page 103.] 



THE perpendicular and oblique veins of flint, found in the chalk cliffs 

 between Brighton and Rottingdean, and described by Dr. Mantell, pre- 

 sent precisely the same internal characters as the tabular flint and the 

 common tuberous nodules. The external characters are also similar to 

 those of the tabular flint. If we observe these veins in situ, we shall 

 frequently perceive, that the whole of their substance is not of uniform 

 density, but that there are often, near the middle of the vein, parts 

 where the two interior surfaces have not united, and that the spaces are 

 generally filled with chalk. If this chalk be carefully removed or dis- 

 solved by diluted muriatic acid, the internal surfaces present the same 

 appearances which have been described as characterizing the exterior of 

 the ordinary flint nodule ; and the aspect of the whole is precisely such 

 as we should expect to find, if the two sides of a fissure in a rock were 

 covered by our common fresh-water sponge, or one of similar habits ; 

 and the two outer surfaces had been built towards each other, and had 

 joined in some parts, while in others they had approached, but had not 

 united. The sides of these flint veins are not studded with numerous 

 species of Foraminiferce, like the under surface of the tabular flint, but 

 from the position in which the flint veins have been built, it is a natural 

 consequence that this should not be the case. 



Having thus satisfied myself that the common tuberous flints, the 

 horizontal tabular flints, and those forming perpendicular or oblique 

 veins, were all produced by the same agency ; and having observed the 

 frequent occurrence of the partial imbedment of shells and other extra- 

 neous bodies, I was naturally led to infer, that in all probability the 

 interior casts of Echinites and similar bodies, which are frequently 

 found to be filled with flint, were also produced by the same agency ; 

 I therefore procured numerous specimens of silicified Echinodermata, 

 and their examination strongly corroborated this supposition. Some of 

 them were not entirely filled with flint ; in one case a small portion only 

 was silicious, in others two-thirds or three-fourths of their interior were 

 so occupied, while the remaining space was filled with chalk. Upon 

 clearing away the chalk from this part of the specimen, the flint never 



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