1872.] SOLLAS-UPPER-GREENSAND COPROLITES. 81
duced to the result. He agreed with Mr. Charlesworth as to the character of the Yorkshire fossil sponges, in which the silica had most
completely replaced the keratose fibres. By the action of acid it was
possible to obtain their skeletons as perfect as those of living species.
There had been no mere infiltration in the case, but a real chemical
union between the silica and the keratose fibres. At Flamborough
Head, close by silicified sponges, he had found others in which the
organic matter had been entirely replaced by oxide of iron instead
of silica; and these, on being treated with acid, were dissolved. In
the case of the Foraminifera, three distinct operations had taken
place — one an infiltration, another the conversion of animal matter
into flint, and the third that of calcareous matter into flint also.
Mr. Sollas in reply pointed out that he had not stated that flint was formed by the silicification of sponge-tissue, but by that of animal matter. He did not think that any mere mineral change could account for the forms of flints. He had only dealt with a limited portion of the question of the origin of flint and coprolites. Of the organic origin of the latter there could be no doubt. He thought that the non-silicified condition of recent dead sponges might be due to a defective supply of silica in the sea-water. He had not put so much stress on dialysis as had been supposed, but relied mainly on the deposition of flint by means of organic matter. From the presence of siliceous spicules of peculiar forms in the coprolites, he could not accept them as of Alcyonarian origin.
vol. xxix. — part I. G
