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.

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).

=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).

2/3 nat. size. From the Silurian of Lilydale, Victoria.

(F.C. Coll.). ]

About 2/3 nat. size. Silurian.

Toongabbie, Gippsland, Victoria.

(Nat. Mus. Coll.) ]

(c) STONE-LILIES.--Ex. Crinoidal or Entrochial limestone, Silurian, Toongabbie, Victoria (Fig. 45). Also the Carboniferous or Mountain limestone, Derbyshire, England.

(d) WORM-TUBES.--Ex. Serpulite limestone of Hanover, Germany. Ditrupa limestone of Torquay and Wormbete Creek, Victoria.

(e) POLYZOA.---Ex. Polyzoal limestone, as the so-called Coralline Crag of Suffolk, England; and the Polyzoal Rock of Mount Gambier, S. Australia.

(f) BRACHIOPODA.--Ex. Brachiopod limestone of Silurian age, Dudley, England. Orthis limestone of Cambrian age, Dolodrook River, N. E. Gippsland.

(g) MOLLUSCA.--Ex. Shell limestone, as the Turritella bed of Table Cape, Tasmania, and of Camperdown, Victoria (Fig. 46), or the Purbeck Marble of Swanage, Dorset, England.

(T. acricula, Tate); 3/4 nat. size. Cainozoic.

Lake Bullen Merri, near Camperdown, Victoria.]

Upper Jurassic. × 9.

Swanage, Dorset, England.]

(h) OSTRACODA.--Ex. Cypridiferous limestone, formed of the minute valves of the bivalved ostracoda, as that of Durlston, Dorset, England (Fig. 47).

(i) CADDIS FLY LARVAE.--Ex. Indusial limestone, formed of tubular cases constructed by the larvae of the Caddis fly (Phryganea). Occurs at Durckheim, Rhine District, Germany.

(j) RED SEAWEEDS.--Ex. Nullipore limestone, formed by the stony thallus (frond) of the calcareous sea-weed Lithothamnion, as in the Leithakalk, a common building stone of Vienna.

(k) GREEN SEAWEEDS.--Ex. Halimeda limestone, forming large masses of rock in the late Cainozoic reefs of the New Hebrides (Fig. 48).

(l) (?) BLUE-GREEN SEAWEEDS.--Ex. Girvanella limestone, forming the Peagrit of Jurassic age, of Gloucester, England.