The Purbeck beds consist of arenaceous, argillaceous, and
calcareous strata, which can be shown by their fossils to consist
of a most remarkable alternation of fresh-water, brackish-water,
and purely marine sediments, together with old land-surfaces,
or vegetable soils, which contain the upright stems of trees,
and are locally known as "Dirt-beds."
One of the most important of the Jurassic deposits of the continent
of Europe, which is believed to be on the horizon of the Coral-rag
or of the lower part of the Upper Oolites, is the "_Solenhofen
Slate_" of Bavaria, an exceedingly fine-grained limestone, which
is largely used in lithography, and is celebrated for the number
and beauty of its organic remains, and especially for those of
Vertebrate animals.
The subjoined sketch-section (fig. 159) exhibits in a diagrammatic
form the general succession of the Jurassic rocks of Britain.
Regarded as a whole, the Jurassic formation is essentially marine;
and though remains of drifted plants, and of insects and other
air-breathing animals, are not uncommon, the fossils of the formation
are in the main marine. In the Purbeck series of Britain,
anticipatory of the great river-deposit of the Wealden, there are
fresh-water, brackish-water, and even terrestrial strata, indicating
that the floor of the Oolitic ocean was undergoing upheaval, and
that the marine conditions which had formerly prevailed were nearly
at an end.
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