220 



THE DAWN OF LIFE. 



with mucli of the foraminiferal slime now accumu- 

 lating in the ocean^ and also with the older deposits 

 of this kind now consolidated in chalks and similar 

 rocks. This name glauconite is_, as Dr. Hunt has 

 shown^ employed to designate not only the hydrous 

 silicate of iron and potash, which perhaps has the 

 best right to it^ but also compounds which contain in 

 addition large percentages of alumina, or magnesia, 

 or both; and one glauconite from the Tertiary lime- 

 stones near Paris, is said to be a trae serpentine, or 

 hydrous silicate of magnesia.* Now the association 

 of such substances with Foraminifera is not purely 

 accidental. Just as a fragment of decaying wood, 

 imbedded in sediment, has the power of decomposing 

 tioluble silicates carried to it by water, and parting 

 Avith its carbon in the form of carbonic acid, in ex- 

 change for the silica, and thus replacing, particle by 

 particle, the carbon of the wood with silicon, so 

 that at length it becomes petrified into a flinty mass, 

 so the sarcode of a Foraminifer, which is a more dense 

 kind of animal matter than is usually supposed, can 

 in like manner abstract silica from the surroundinsr 

 water or water-soaked sediment. From some pecu- 

 liarity in the conditions of the case, however, our 

 Protozoon usually becomes petrified with a hydrous 

 silicate instead of with pure silica. The favourable 

 conditions presented by the deep sea for the combina- 

 tion of silica with bases, may perhaps account in part 



* Berthier, quoted by Hunt. 



