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enveloped, a portion of the shell had been replaced by silica.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 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.
Rev. O. Fisher did not understand how dialysis could have segregated 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. 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 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 silicification. 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 remarks 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-
