_Thickness of the Wealden._—In the Weald area itself, between the North and South Downs, fresh-water beds to the thickness of 1600 feet are known, the base not being reached. Probably the thickness of the whole Wealden series, as seen in Swanage Bay, cannot be estimated as less than 2000 feet. _Wealden Flora._—The flora of the Wealden is characterised by a great abundance of Coniferæ, Cycadeæ, and Ferns, and by the absence of leaves and fruits of Dicotyledonous Angiosperms. The discovery in 1855, in the Hastings beds of the Isle of Wight, of Gyrogonites, or spore-vessels of the Chara, was the first example of that genus of plants, so common in the tertiary strata, being found in a Secondary or Mesozoic rock. [1] Judd, Speeton clay, Quart. Geol. Journ., vol. xxiv, 1868, p. 218. [2] Judd, Quart. Geol. Journ., 1867, vol. xxiii, p. 249. [3] Mantell, Geol. of S.E. of England, p. 244. [4] Foss. de Utrillas. [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.