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These beds are for the most part of fresh-water origin, but the organic remains of some few intercalated beds are marine, and show that the Purbeck series has a closer affinity to the Oolitic group, of which it may be considered as the newest or uppermost member.
Having arrived at the hills, in about three miles, we found them abundantly grassed, but very rugged and rocky, of an oolitic limestone formation, with occasionally a light reddish soil covering the rock in the flats and valleys.
Upon crossing this region deep gorges or valleys are met with, through which flow brackish or salt-water streams, and shading these are found the tea-tree and the bastard gum. The steep banks which inclose the valleys, through which the streams take their course, and which until lately we had found of an oolitic limestone, now exhibited granite, quartz, sandstone or iron-stone. June 23.
The alternation, on a grand scale, of distinct formations of clay and limestone has caused the oolitic and liassic series to give rise to some marked features in the physical outline of parts of England and France.
Hooker, that there are Carboniferous plants which appear to be generically identical with some now living; that the cone of the Oolitic Araucaria is hardly distinguishable from that of an existing species; that a true Pinus appears in the Purbecks and a Juglans in the Chalk; while, from the Bagshot Sands, a Banksia, the wood of which is not distinguishable from that of species now living in Australia, had been obtained.
Three miles from where we had halted during the heat of the day, we passed some tolerable grass, though dry, scattered at intervals among the scrub, which grew here in dense belts, but with occasional openings between. The character of the ground was very rocky, of an oolitic limestone, and having many hollows on its surface.
On the island beaches coral sand is forming oolitic limestone, and the white coral mud with which the sea is milky for miles about the reef in times of storm settles and concretes into a compact limestone of finest grain. Corals have been among the most important limestone builders of the sea ever since they made their appearance in the early geological ages.
Their rock formation consisted of various strata, the upper crust or surface being an oolitic limestone; below this is an indented concrete mixture of sand, soil, small pebbles, and shells; beneath this appear immense masses of a coarse greyish limestone, of which by far the greater portion of the cliffs are composed; and immediately below these again is a narrow stripe of a whitish, or rather a cream-coloured substance, lying in horizontal strata, but which the impracticable nature of the cliffs did not permit me to examine.
While himself asserting that the Potsdam-sandstone of North America, the Lingula-flags of England, and the alum-slates of Scandinavia are of the same period while fully aware that among the Silurian formations of Wales, there are oolitic strata like those of secondary age; yet his reasoning is more or less coloured by the assumption, that formations of like qualities probably belong to the same era.
In connexion with this circumstance, it is a fact of some importance, that the geognostic character of Australia, its vast arid plains, its little diversified surface and consequent paucity of streams, and the very slight development of volcanic rock on its surface, seem to indicate a system of physical conditions, such as we may suppose to have existed elsewhere in the oolitic era: perhaps we see the chalk formation preparing there in the vast coral beds frontiering the coast.
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