Geological Society of London. 93 



the state of their silica ofiFered arguments tending to prove a passage of silica from 



the colloidal to the crystalline state. 



The second part of the paper discussed the Coprolites specially; their exterior 

 appearance is extremely sponge-like, almost exactly resembling some species of 

 modern sponges. They are marked by oscules of peculiar characters. 



The so-called "pores" of palajontologists are well marked. Spicules, triradiate, 

 hexradiate, sinuous, defensive and connecting, have been observed. They are siliceous 

 in composition. On dissolving the coprolites in acid, the spicules are set free, 

 associated with Pohjcystina {Raliomma hexacantha, etc.) and Xanthidia (iV. furcatnm). 

 The genera and species of coprolites described were as follows : — Rhabdospongia 

 communis, Bonneyia bacillifoDiiis, B. cyliitdricus, B. Jessoni, B. scrobiculatus, 

 B. verrongiformis, Acantliophora Sartogii, Bolyeantha JStheridgii, Metia simplex, 

 M. costata, Ulospongia 2mtera, U. calyx, U. Brunii. The external appearance of 

 these forms, which constitute a great number of the coprolites, their curious oscules 

 and siliceous spicules, were said to leave no doubt as to their spongious origin. 



Discussion. — Mr. Charlesworth complained that the author had not fully stated 

 Dr. Bowerbauk's views, which were founded on the fact that flint, wherever found, 

 whether in iissures, the interior of organisms, or elsewhere, always presented under 

 the microscope a reticulated structure. He had himself combated the view that flint 

 ■was in all cases silicified sponge, and had demonstrated that certainly, in some cases, 

 flint had been formed without the intervention of sponges ; for in the case of the 

 teeth in a lower jaw of a Mosasaurus found in the chalk, he had found the pulp- 

 cavities completely filled with black flint. At the same time the bone and dentine 

 had remained unsilicified. Even in this flint, however, Dr. Bowerbank thought 

 he had recognized spongy texture, and accounted for the presence of sponge in 

 so singular a position in what seemed to be by no means a satisfactory manner. 

 Mr. Charlesworth maintained that siliceous matter did not always follow organic 

 fibre, and that in the case of most Ventriculites the fiint never embraced the whole of 

 the organism. In the chalk of the southern parts of England, the roots and upper 

 portion of Ventriculites were hardly ever to his knowledge completely silicified. In 

 Yorkshire, on the contrary, the whole body of the sponge was silicified, and flint was 

 but rarely found of other forms. He called attention to the fact that when a shell, 

 such as that of an Echinoderm, was completely filled and enveloped by flint, it 

 remained in the state of carbonate of lime ; where only flUed but not enveloped, 

 a portion of the shell had been replaced by silica. 



Prof. T. Rupert Jones regretted that observations in this country were principally 

 confined to the flints of our Southern Chalk. He had himself never seen such 

 silicified shells of Echinoderms as those described by Mr. Charlesworth. He 

 considered that in many cases the flint was, in fact, pseudomorphous silica after 

 amorphous carbonate of lime, and that there was a gradual change from carbonate of 

 lime into silica taking place, so that the theory of the author was not in all 

 cases applicable. 



Eev. 0. Fisher was not prepared to accept the whole of the coprolites as having 

 been originally sponges, nor as having been derived from the Gault. He thought, on 

 the contrary, that they came out of a chloritic marl which was not truly Upper 

 Greensand. 



Mr. Carruthers thought that the speculations in the first part of the paper, though 

 interesting, formed only a portion of a very large subject; and he would be glad to 

 see the considerations extended to the fossilization by silica of other bodies than 

 sponges. He thought in most instances the silicification was the result of what took 

 place long after the organisms had been imbedded in the rock. 



Mr. Gwyn JeS"reys stated that in deep-sea explorations he had found both siliceous 

 and calcareous sponges in the same area. He had taken them both alive and dead, 

 and in no case was there silicification. In the case of foraminifera he had, however, 

 found the interior filled with silica. 



Mr. Johnson stated that he had not confined his remark? in the paper cited by the 

 author to the substitution of silicon for carbon, but had instanced that as only one of 

 the steps towards silicification. 



Mr.. Evans mentioned the late M. Meillet, of Poitiers, as having some years since 

 pointed out the cause of the whitening of fiint by age. 



Mr. Woodward pointed out that the condition of most silicified Ventriculites seems 

 to aff'ord evidence that the process had gone on at a period long subsequent to their 

 imbedding in the Chalk. 



