[5] Fitton, Geol. of Hastings, p. 58, who cites Lander’s Travels.
[6] See [p. 102] and Second Visit to the United States, vol. ii, chap. xxxiv.
CHAPTER XIX.
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 of Dorsetshire. — Dirt-bed or ancient Soil. — Fossils of the Purbeck Beds. — Portland Stone and Fossils. — Kimmeridge Clay. — Lithographic Stone of Solenhofen. — Archæopteryx. — Middle Oolite. — Coral Rag. — Nerinæa Limestone. — Oxford Clay, Ammonites and Belemnites. — Kelloway Rock. — Lower, or Bath, Oolite. — Great Plants of the Oolite. — Oolite and Bradford Clay. — Stonesfield Slate. — Fossil Mammalia. — Fuller’s Earth. — Inferior Oolite and Fossils. — Northamptonshire Slates. — Yorkshire Oolitic Coal-field. — Brora Coal. — Palæontological Relations of the several Subdivisions of the Oolitic group.
Classification of the Oolite.—Immediately below the Hastings Sands we find in Dorsetshire another remarkable fresh-water formation, called the Purbeck, because it was first studied in the sea-cliffs of the peninsula of Purbeck in that county. These beds are for the most part of fresh-water origin, but the organic remains of some few intercalated beds are marine, and show that the Purbeck series has a closer affinity to the Oolitic group, of which it may be considered as the newest or uppermost member.
In England generally, and in the greater part of Europe, both the Wealden and Purbeck beds are wanting, and the marine cretaceous group is followed immediately, in the descending order, by another series called the Jurassic. In this term, the formations commonly designated as “the Oolite and Lias” are included, both being found in the Jura Mountains. The Oolite was so named because in the countries where it was first examined the limestones belonging to it had an Oolitic structure (see [p. 37]). These rocks occupy in England a zone nearly thirty miles in average breadth, which extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral characters are not uniform throughout this region; but the following are the names of the principal subdivisions observed in the central and south-eastern parts of England.
| Upper | a. Purbeck beds. b. Portland stone and sand. c. Kimmeridge clay. |
| Middle | d. Coral rag. e. Oxford clay, and Kelloway rock. |
| Lower | f. Cornbrash and Forest marble. g. Great Oolite and Stonesfield slate. h. Fuller’s earth. i. Inferior Oolite. |
The Upper Oolitic system of the above table has usually the Kimmeridge clay for its base; the Middle Oolitic system, the Oxford clay. The Lower system reposes on the Lias, an argillo-calcareous formation, which some include in the Lower Oolite, but which will be treated of separately in the next chapter. Many of these subdivisions are distinguished by peculiar organic remains; and, though varying in thickness, may be traced in certain directions for great distances, especially if we compare the part of England to which the above-mentioned type refers with the north-east of France and the Jura Mountains adjoining. In that country, distant above 400 geographical miles, the analogy to the accepted English type, notwithstanding the thinness or occasional absence of the clays, is more perfect than in Yorkshire or Normandy.
Physical Geography.—The alternation, on a grand scale, of distinct formations of clay and limestone has caused the oolitic and liassic series to give rise to some marked features in the physical outline of parts of England and France. Wide valleys can usually be traced throughout the long bands of country where the argillaceous strata crop out; and between these valleys the limestones are observed, forming ranges of hills or more elevated grounds. These ranges terminate abruptly on the side on which the several clays rise up from beneath the calcareous strata.