Oh. XX.]

DIVISIONS OF THE OOLITE,

375

CHAPTER XX.

JURASSIC GROUP_PURBECK BEDS AND OOLITE.

The Purbeck beds a member of the Jurassic group—Subdivisions of that group—Physical geography of the Oolite in England and France —Upper Oolite—•Purbeck beds—New genera of fossil mammalia in the Middle Purbeck ofDorsetshire —Dirt-bed or ancient soil—Fossils of the Purbeck beds—Portlandstone and fossils—Lithographic stone of Solenhofen —Archaeopteryx—MiddleOolite—Coral rag—Zoophytes—Nerinaean limestone—Diceras limestone—Ox-ford clay, Ammonites and Belemnites—Kelloway Rock—Lower Oolite, Cri-noideans—Great Oolite and Bradford clay—Stonesfield slate—Fossil mammalia•—Resemblance to an Australian fauna—Northamptonshire slates—Yorkshire Oolitic coal-field—Brora coal—Fuller’s earth—Inferior Oolite and fossils—Palae-ontological relations of the several subdivisions of the Oolitic group.

Immediately below the Hastings Sands (the inferior member of theWealden, as defined in the eighteenth chapter), we find in Dorsetshire ,another remarkable freshwater formation, called the Purbeck, be-cause it was first studied in the sea-cliffs of the peninsula of Pur-beck in Dorsetshire . These beds were formerly grouped with theWealden, but some organic remains recently discovered in certainintercalated marine beds show that the Purbeck series has a closeaffinity to the Oolitic group, of which it may be considered as thenewest or uppermost member.

In England generally, and in the greater part of Europe , boththe Wealden and Purbeck beds are wanting, and the marinecretaceous group is followed immediately, in the descending order,by another series called the Jurassic . In this term, the formationscommonly designated as “ the Oolite and Lias ” are included, bothbeing found in the Jura Mountains . The Oolite was so named be-cause in the countries where it was first examined, the limestonesbelonging to it had an oolitic structure (see p. 12.). These rocksoccupy in England a zone which is nearly thirty miles in averagebreadth, and extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral charactersare not uniform throughout this region ; but the following are thenames of the principal subdivisions observed in the central andsouth-eastern parts of England.

OOLITE.

f a. Purbeck beds.

Upper j b. Portland stone and sand.[ c. Kimmeridge clay.