262 RELICS OF PRIMEVAL LIFE 



decaying wood, imbedded in sediment, has the 

 [jower of decomposing soluble silicates carried to 

 it by water, and parting with its carbon in the form 

 of carbonic acid, in exchange for the silica, and thus 

 replacing, particle by particle, the carbon of the woo ; 

 with silicon, so that at length it becomes petrified 

 into a flinty mass, so the sarcode of a Foraminifer 

 can in like manner abstract silica from the surround- 

 ing water or water-soaked sediment. From some 

 peculiarity 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 combination of silica with bases, as indicated in 

 the reports of the Challenger already referred to, may 

 perhaps account in part for this. But whatever the 

 cause, it is usual to find fossil Foraminifera with 

 their sarcode replaced by such material. We also 

 find beds of glauconite retaining the forms of Fora- 

 minifera, while the calcareous tests of these have 

 been removed, apparently by acid waters. 



One consideration which, though conjectural, de- 

 serves notice, is connected with the food of these 

 humble animals. They are known to feed to a large 

 extent on minute plants, the Diatoms, and other 



