-. 225 HISTORICAL PALAZONTOLOGY. 
(‘“‘ Portland Stone”), the whole series attaining a thickness of 
150 feet or more, and containing marine fossils; ¢, The Pur- 
beck 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 alter- 
nation of fresh-water, brackish-water, and purely marine sedi- 
ments, 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 BE 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 cele- 
brated 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 dia- 
grammatic 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 terres- 
trial 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 indu- 
bitable sea-deposit ; and its limestones, oolitic as they com- 
monly are, nevertheless are composed largely of the commin- 
uted 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 ob- 
servers 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 
