1872.] SOLLAS TTPPEK-GEEENSAin) COPKOLITES. 81 



duced to the result. He agreed with Mr. Charlesworth as to the cha- 

 racter 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 r-elied 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 r. 



