Fig. 123.—Table to indicate distribution of Sponges in time.

Flints.—The ultimate source of all the silica in the sea and fresh-water areas is to be found in the decomposition of igneous rocks such as granite. The quantity of silica present in solution in sea water is exceedingly small, amounting to about one-and-a-half parts in 100,000; it certainly is not much more in average fresh water. This is no doubt due to its extraction by diatoms, which begin to extract it almost as soon as it is set free from the parent rock. It is from this small quantity that the siliceous sponges derive the supply from which they form their spicules. Hence it would appear that for the formation of one 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 Radiolaria. 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 Textularia and those of Coccoliths, are converted into a siliceous chalk. Thus a siliceous chalk is the first stage 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 subsequent 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.[[282]]

COELENTERATA AND CTENOPHORA

BY

S. J. HICKSON, M.A., F.R.S.

Formerly Fellow and now Honorary Fellow of Downing College, Beyer Professor of Zoology in the Victoria University of Manchester.

CHAPTER X

COELENTERATA