242 



PORIFERA 



ounce of spicules at least one ton of sea water must pass through 

 the body of the sponge. Obviously from such a weak solution 

 the deposition of silica will not occur by ordinary physical 

 agencies ; it requires the unexplained action of living organisms. 

 This may account for the fact that deposits of flint and chert 

 are always associated with organic remains, such as Sponges and 

 Eadiolaria. By some process, the details of which are not yet 

 understood, the silica of the skeleton passes into solution. In 

 Calcareous deposits, a replacement of the carbonate of lime by 

 the silica takes place, so that in the case of chalk the shells of 

 Foraminifera, such as Globigerina and Tcxtularia and those of 

 Coccoliths, are converted into a siliceous chalk. Thus a siliceous 

 chalk is the first sta^e in the formation of a flint. 



A further deposition of silica then follows, cementing this 

 pulverulent material into a hard white porous flint. It is white 

 for the same reason that snow is white. The deposition of silica 

 continues, and the flint becomes at first grey and at last apparently 

 black (black as ice is black on a pond). Frequently flints are found 

 in all stages of formation: siliceous chalk with the corroded remains 

 of sponge spicules may be found in the interior, black flint blotched 

 with grey forming the mass of the nodule, while the exterior is 

 completed by a thin layer of white porous flint. This layer must 

 not be confused with the white layer which is frequently met 

 with on the surface of weathered flints, which is due to a sub- 

 sequent solution of some of the silica, so that by a process of 

 unbuilding, the flint is brought back to the incompleted flint in 

 its second stage. In the chalk adjacent to the flints, hollow 

 casts of large sponge spicules may sometimes be observed, proving 

 the fact, which is however unexplained, of the solution of the 

 spicular silica. The formation of the flints appears to have taken 

 place, to some extent at least, long after the death of the sponge, 

 and even subsequent to the elevation of the chalk far above the 

 sea-level, as is shown by the occurrence of layers of flints in the 

 joints of the solid chalk. 1 



1 For further details see Sollas, "The Formation of Flints," in The Age of the 

 Earth, 1905, p. 131. 



