80 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 18, 



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 flint 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 ; when only filled 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 enveloped, a portion of the shell had been replaced by silica.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 car- 

 bonate 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. 



Rev. 0. Eisher did not understand how dialysis could have segre- 

 gated a mineral from a solution surrounding an immersed body on 

 all sides. He thought the coprolites had not been derived from the 

 Gault, but that they came out of a chloritic marl which was not 

 truly Upper Greensand. 



Mr. Carrttthers 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 Jeffreys 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 silici- 

 fication. In the case of Foraminifera, however, he had found the 

 interior filled with silica, evidently by infiltration. 



Mr. Hawkins Johnson stated that he had not confined his re- 

 marks 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 flint by age. 



Mr. Woodward pointed out that the condition of most silicified 

 Ventriculites seems to afford evidence that the process had gone on 

 at a period long subsequent to their being imbedded in the Chalk. 



Prof. Williamson commented on the difficulties of the case, and 

 said he believed that in most fossils more than one process had con- 



