796 STRATIGRAPHICAL GEOLOGY. — [Boox VI. _ 
(Steneosaurus, Ceteosaurus) still appear, together with the crocodile 
Goniopholis.' 
(3.) Purbeckian.—This group, so named from the Isle of Purbeck, 
where best developed, is usually connected with the foregoing forma- 
tions as the highest zone of the Jurassic series of England. But it is 
certainly separated from the rest of that series by many peculiarities, 
which show that it was accumulated at a time when the physical geo- 
graphy and the animal and vegetable life of the region were undergoing 
a remarkable change. The Portland beds were gently upraised and 
even somewhat denuded before the lowest Purbeckian strata were de- - 
posited. Hence a considerable stratigraphical and paleontological 
break is to be remarked at this line. The sea-floor was converted partly 
into land, partly into shallow estuaries. The characteristic marine 
fauna of the Jurassic seas nearly disappeared from the area, its place 
being taken by fresh-water and brackish-water forms. 
The Purbeckian beds have been divided into three sections. Of 
these the lowest consists of fresh-water limestones and clays, with layers 
of ancient soil (‘‘dirt beds ’’) containing stumps of the trees which grew in 
them ; the middle comprises about 130 feet of strata with marine fossils, 
while the highest shows a return of fresh-water conditions. Among the 
indications of the presence of the sea is an oyster-bed (Ostrea distorta) 
12 feet thick, with Pecten, Modiola, Avicula, Thracia, &c. The fresh-water 
bands contain still livi ing genera of lacustrine and fluviatileshells (Paludina, 
Limnza, Planorbis, Physa, Valvata, Unio, and Cyclas). Numerous fishes, 
both placoid and ganoid, haunted these Purbeck waters. Many insects, 
blown off from the adjacent land, sank and were entombed and preserved 
in the calcareous mud. ‘These include coleopterous, orthopterous, hemi- 
pterous, neuropterous, and dipterous forms (Fig. 376). Remains of 
several reptiles, chiefly chelonian, but including the old Jurassic croco- 
dile Goniopholis, have also been discovered. ‘The most remarkable 
organisms of this group of strata are the mammalian forms already ~ 
noticed (p. 782). It is deserving of note that these remains occur, 
almost wholly as lower jaws, in a stratum about 5 inches thick lying 
near the base of the Middle Purbeck group, these being the portions of 
the skeleton that would be most likely first to drop out of floating 
and decomposing carcases. 
France, &c.—The Jurassic system is here symmetrically developed in 
the form of two great connected rings. The southern ring encloses the 
crystalline axis of the centre and south; the northern and larger ring 
encircles the Cretaceous and Tertiary basin and opens towards the Channel, 
where its separated ends point across to the continuation of the same 
rocks in England. But the structure of the two areas is exactly opposite, 
for in the southern area the oldest rocks lie in the centre and the 
Jurassic strata dip outwards, while in the northern region the youngest 
formations lie in the centre and the Jurassic beds dip inward below them. 
Where the two rings unite in the middle of France they send a tongue 
down to the Bay of Biscay. On the eastern side of the country the 
Jurassic system is copiously developed, and extends thence eastwards 
through the Jura Mountains into Germany. 
The subdivisions of the Jurassic system in the north and north-west 
of France resemble generally those established in England, but gradually 
1 J. F. Blake, Op. cit. 
