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 observers as Quenstedt,
Oppel, D'Orbigny, Wright, De la Beche, Tate, and others, the
entire series of Jurassic sediments admits of a more complete
and more elaborate subdivision into zones characterised by special
life-forms than has as yet been found practicable in the case
of any other rock-series.
[Illustration: Fig. 159. GENERALIZED SECTION OF THE JURASSIC ROCKS
OF ENGLAND.]
[Illustration: Fig. 160.--_Mantellia_ (_Cycadeoidea_) _megalophylla_,
a Cycad from the Purbeck "dirt-bed." Upper Oolites, England.]
The _plants_ of the Jurassic period consist principally of Ferns,
Cycads, and Conifers--agreeing in this respect, therefore, with
those of the preceding Triassic formation.
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