Section Iv.--Carbonaceous and Miscellaneous Rocks.

COALS and KEROSENE SHALES (Cannel Coal).--These carbonaceous rocks are formed in much the same way as the deposits in estuaries and lagoon swamps. They result from the sometimes vast aggregation of vegetable material (leaves, wood and fruits), brought down by flooded rivers from the surrounding country, which form a deposit in a swampy or brackish area near the coast, or in an estuary. Layer upon layer is thus formed, alternating with fine mud. The latter effectually seals up the organic layers and renders their change into a carbonaceous deposit more certain.

When shale occurs between the coal-layers it is spoken of as the under-clay, which in most cases is the ancient sub-soil related to the coal-layer immediately above. It is in the shales that the best examples of fossil ferns and other plant-remains are often found. The coal itself is composed of a partially decomposed mass of vegetation which has become hardened and bedded by pressure and gradual drying.

Spore coals are found in thick deposits in some English mines, as at Burnley in Yorkshire. They result from the accumulation of the spores of giant club-mosses which flourished in the coal-period. They are generally referred to under the head of Cannel Coals. The "white coal" or Tasmanite of the Mersey Basin in Tasmania is an example of an impure spore coal with a sandy matrix (Fig. 49).

=Rock composed of the calcareous joints of Halimeda (a green sea-weed).=

About 2/3 nat. size. Late Cainozoic. Reef-Rock. Malekula, New Hebrides.

(Coll. by Dr. D. Mawson.) ]

× 28. Carbopermian. Latrobe, Tasmania.

(F. C. Coll.) ]

× 28. Carbopermian. Hartley, New South Wales.

(F. C. Coll.) ]

About 1/2 nat. size. (Rhaetic). Aust Cliff, Gloucestershire, England.

(Nat. Mus. Coll.) ]

The Kerosene Shale of New South Wales is related to the Torbanite of Scotland and Central France. It occurs in lenticular beds between the bituminous coal. It is a very important deposit, commercially speaking, for it yields kerosene oil, and is also used for the manufacture of gas. The rock is composed of myriads of little cell-bodies, referred to as Reinschia, and first supposed to be allied to the freshwater alga, Volvox; but this has lately been questioned, and an alternative view is that they may be the megaspores of club-mosses (Fig. 50).

The coals of Jurassic age in Australia are derived from the remains of coniferous trees and ferns; and some beautiful examples of these plants may often be found in the hardened clay or shale associated with the coal seams.

The Brown Coals of Cainozoic or Tertiary age in Australia are still but little advanced from the early stage, lignite. The leaves found in them are more or less like the present types of the flora. The wood is found to be of the Cypress type (Cupressinoxylon). In New Zealand, however, important deposits of coal of a more bituminous nature occur in the Oligocene of Westport and the Grey River Valley, in the Nelson District.

BONE BEDS.--The bones and excreta of fish and reptiles form considerable deposits in some of the sedimentary formations; especially those partly under the influence of land or swamp conditions. They constitute a kind of conglomerate in which are found bone-fragments and teeth (Fig. 51). These bone-beds are usually rich in phosphates, and are consequently valuable as a source of manure. The Miocene bone-bed with fish teeth at Florida, U.S.A., is a notable example. The nodule bed of the Victorian Cainozoics contains an assemblage of bones of cetaceans (whales, etc.).

About 3/4 nat. size. Pleistocene.

Limeburners Point, Geelong, Victoria.

(Nat. Mus. Coll.) ]

BONE BRECCIAS.--These are usually formed of the remains of the larger mammals, and consist of a consolidated mass of fragments of bones and teeth embedded in a calcareous matrix. Bone-breccias are of frequent occurrence on the floors of caves which had formerly been the resort of carnivorous animals, and into which they dragged their prey. The surface water percolating through the overlying calcareous strata dissolved a certain amount of lime, and this was re-deposited on the animal remains lying scattered over the cave floor. A deposit so formed constitutes a stalagmite or floor encrustation. As examples of bone-breccias we may refer to the limestone at Limeburners Point, Geelong (Fig. 52); and the stalagmitic deposits of the Buchan Caves.

IRONSTONE.--Rocks formed almost entirely of limonite (hydrated peroxide of iron) are often due to the agency of unicellular plants known as diatoms, which separate the iron from water, and deposit it as hydrous peroxide of iron within their siliceous skeletons. In Norway and Sweden there are large and important deposits of bog iron-ore, which have presumably been formed in the beds of lakes.

=Cainozoic Ironstone with Leaves (Banksia ? marginata, Cavanilles).=

Slightly enlarged. Below Wannon Falls, Redruth, Victoria.]

Clay ironstone nodules (sphaerosiderite) have generally been formed as accretions around some decaying organic body. Many clay ironstone nodules, when broken open, reveal a fossil within, such as a coprolitic body, fern frond, fir-cone, shell or fish.

Oolitic ironstones are composed of minute granules which may have originally been calcareous grains, formed by a primitive plant or alga, but since replaced by iron oxide or carbonate.

The Tertiary ironstone of western Victoria is found to contain leaves, which were washed into lakes and swamps (Fig. 53); and the ferruginous groundmass may have been originally due to the presence of diatoms, though this yet remains to be proved.