es ("Portland Stone"),
the whole series attaining a thickness of 150 feet or more, and
containing marine fossils; c, The _Purbeck_ Beds are apparently
peculiar to Great Britain, where they form the summit of the entire
Oolitic series, attaining a total thickness of from 150 to 200
feet. 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. In places also, as in Yorkshire and Sutherlandshire,
are found actual beds of coal: but the great bulk of the formation
is an indubitable sea-deposit; and its limestones, oolitic as
they commonly are, nevertheless are composed largely of the
comminuted skeletons of marine animals. Owing to the enormous
number and variety of the organic remains which have been yielded
by the richly fossiliferous strata of the Oolitic series, it will
not be possible here to do more than to give an outline-sketch
of the principal forms of life which characterise the Jurassic
period as a whole. It is to be remembered, however, that every
minor group of the Jurassic formation has its own peculiar fossils,
and that by the labours of such eminent observ