Silica (which is required for the skeletons of diatoms, radiolaria, peridinians, etc.) is present in about the same concentration, but it is now suspected that a source of this substance may be clay washed down from the land and present in the sea in the colloidal form.
Classes: Rhizopoda, Grega Rinida, Radiolaria, Spongida.
The limestones contain Globigerina and other Foraminifera, the siliceous beds are made of Radiolaria, sponge spicules and diatoms, while the red clay closely resembles the red clay of the deepest parts of the oceans.
Radiolaria are found in cherts in the Culm of Devonshire and Cornwall, in Russia, Germany and elsewhere.
Simple protozoa appear in the form of Radiolaria; Lithistid sponges are represented by such forms as Archaeoscyphia, Hexactinellid sponges by Protospongia; Graptolites (Dictyograptus (Dictyonema)) come on in the higher parts of the system.
The silica was derived from the tests of radiolaria and the spicular skeletons of sponges.
Some cherts contain tests of radiolaria, and correspond fairly closely to the siliceous radiolarian oozes which are gathering at the present day at the bottom of some of the deepest parts of the oceans.
The only fossils of the clay are radiolaria, sharks' teeth and the ear-bones of whales, precisely those parts of the skeleton of marine creatures which are hardest and can longest survive exposure to sea-water.