II 





iir: 



170 RELICS OF rklMEVAL LIFE 



the Calcaire Grossier of Paris analysed by Berthier* 

 is essentially a serpentine composed of silicate of 

 iron and magnesia, that Loganite as analysed by 

 Hunt contains thirty-one per cent, of magnesia, 

 and that Hoskins has shown * that modern glau- 

 conites often contain large proportions of magr sia 

 and equivalent bases. 



It is also to be observed that independently of 

 volcanic debris the reports of the Challenger ex- 

 pedition show that in the deep seas the decay of 

 organic matter causes an alkaline condition of the 

 sediments leading to the formation of alkaline 

 silicates, while the presence of decaying volcanic 

 dust furnishes the basis, whether of iron, alumina, 

 or magnesia, necessary for the making up of 

 glauconite. I have also suggested that the assimila- 

 tion by Protozoa making calcareous skeletons, of the 

 111 matter of Diatoms or humble plants having soluble 



silica in their organization or of silicious Protozoa, 

 ii and sponge germs, must set free much soluble 



silica as a rejected or excremPiititious matter which 

 may contribute to the same result. 



* Beudant, Mineralogiey xi. 178. 

 ■ Geological Magazine^ July, 1895. 



