Section II.—SILICEOUS ROCKS.
In this group are comprised all granular quartzose sediments, and organic rocks of flinty composition.
SANDSTONES.—Although the base of this type of rock is formed of quartz sand, it often contains fossils. Owing to its porous nature, percolation of water containing dissolved CO2 tends to bring about the solution of the calcareous shells, with the result that only casts of the shells remain.
FLINTS and CHERTS.—These are found in the form of nodules and bands in other strata, principally in limestone. In Europe, flint is usually found in the Chalk formation, whilst chert is found in the Lower Greensands, the Jurassics, the Carboniferous Limestone and in Cambrian rocks. In Australia, flint occurs in the Miocene or Polyzoal-rock formation of Mount Gambier, Cape Liptrap and the Mallee borings. Flint is distinguished from chert by its being black in the mass, often with a white crust, and translucent in thin flakes; chert being more or less granular in texture and sub-opaque in the mass. Both kinds appear to be formed as a pseudomorph or replacement of a portion of the limestone stratum by silica, probably introduced in solution as a soluble alkaline silicate. Both flint and chert often contain fossil shells and other organic remains, such as radiolaria and sponge-spicules, which can be easily seen with a lens in thin flakes struck off by the hammer.
DIATOMITE is essentially composed of the tiny frustules or flinty cases of diatoms (unicellular algae), usually admixed with some spicules of the freshwater sponge, Spongilla. It generally forms a layer at the bottom of a lake bed ([Fig. 42]).
Fig. 42—Diatomaceous Earth. (Post-Tertiary).
Containing freshwater forms, as Pinnularia, Cocconeis and
Synedra. × 150. Talbot, Victoria.
Section III.—CALCAREOUS ROCKS.