SLATE.--When shale is subjected to great pressure, a plane of regular splitting called cleavage is induced, which is rarely parallel to the bedding plane or surface spread out on the original sea-floor: the cleavage more often taking place at an appreciable angle to the bedding plane. The graptolitic rocks of Victoria are either shales or slates, according to the absence or development of this cleavage structure in the rock. 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 CO_{2} 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). [Illustration: =Fig. 42.--Diatomaceous Earth. (Post-Tertiary).= Containing freshwater forms, as Pinnularia, Cocconeis and Synedra. × 150. Talbot, Victoria.] Section III.--CALCAREOUS ROCKS. LIMESTONES FORMED BY ORGANISMS.--Organic limestones constitute by far the most important group of fossiliferous rocks. Rocks of this class are composed either wholly of carbonate of lime, or contain other mineral matter also, in varying proportion. Many kinds of limestones owe their origin directly to the agency of animals or plants, which extracted the calcareous matter from the water in which they lived in order to build their hard external cases, as for example the sea-urchins; or their internal skeletons, as the stony corals. The accumulated remains of these organisms are generally compacted by a crystalline cement to form a coherent rock.