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.
The chief groups of animals and plants forming such limestone rocks are:—
(a) FORAMINIFERA.—Example. Foraminiferal limestone as the Nummulitic limestone of the Pyramids of Egypt, or the Lepidocyclina limestone of Batesford, near Geelong, Victoria ([Fig. 43]).
Fig. 43.
Limestone composed of Polyzoa and Foraminifera (Lepidocyclina).
× 6. Cainozoic (Janjukian). Batesford, near Geelong, Victoria.
(F.C. Coll.)
(b) CORALS.—Ex. “Madrepore limestone,” or Devonian marble, with Pachypora. Also the Lilydale limestone, with Favosites, of Silurian age, Victoria ([Fig. 44]).