Ch. XX.] DIVISIONS OF THE OOLITE. 377 



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 of Dorsetshire — 

 Dirt-bed or ancient soil — Fossils of the Purbeck beds — Portland stone and fossils 

 — Lithographic stone of Solenhofen — Archteopteryx — Middle Oolite — Coral rag — 

 Zoophytes — Nerineean limestone — Diceras limestone — Oxford clay, Ammonites, 

 and Belemnites — Kelloway Rock — Lower Oolite, Crinoideans — Great Oolite and 

 Bradford clay — Stonesfield slate — Fossil mammalia — Resemblance to an Austra- 

 lian fauna — Northamptonshire slates — Yorkshire Oolitic coal-field — Brora coal — 

 Fuller's earth — Inferior Oolite and fossils — Palaeontological relations of the sev- 

 eral subdivisions of the Oolitic group. 



Immediately below the Hastings Sands (the inferior member of the 

 Wealden, as defined in Chapter XVIIL), we find in Dorsetshire, an- 

 other remarkable freshwater formation, called the Purbeck, because it 

 was first studied in the sea-cliffs of the peninsula of Purbeck in Dorset- 

 shire. These beds were formerly grouped with the "Wealden, but some 

 organic remains recently discovered in certain intercalated marine beds 

 show that the Purbeck series has a close 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 ex- 

 amined, the limestones belonging to it had an oolitic structure (p. 12). 

 These rocks occupy in England a zone which is nearly 30 miles in aver- 

 age breadth, and extends across the island, from Yorkshire in the north- 

 east, to Dorsetshire in the southwest. Their mineral characters are not 

 uniform throughout this region ; but the following are the names of the 

 principal subdivisions observed in the central and southeastern parts 



of England : 



OOLITE. 



\\ 



Purbeck beds. 

 Upper \ b. Portland stone and sand. 



Kimmeridge clay. 

 -,,. , ,, j d. Coral rag. 



I e. Oxford clay, and Kelloway rock. 



(f. Cornbrash and Forest marble. 

 T j a. Great Oolite and Stonesfield slate. 



Lower i I Fuller's earth. 



(_i. Inferior Oolite. 

 The Lias then succeeds to the Inferior Oolite. 



