(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.

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]).

Fig. 48.
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.) 		Fig. 49—Thin Slices of “White
Coal” or “Tasmanite,” showing
crushed Megaspores.
× 28. Carbopermian. Latrobe, Tasmania.
(F. C. Coll.)
Fig. 50—Thin Slice of
“Kerosene Shale.”
× 28. Carbopermian. Hartley, New South Wales.
(F. C. Coll.) 		Fig. 51—Bone Bed, with Fish and
Reptilian Remains.
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.