Oh. XX.] DIVISIONS OF THE OOLITE. 291 



CHAPTER XX. 



JURASSIC GROUP. PURBECK BEDS AND OOLITE. 



The Purbeck beds a member of tlie Jurassic group — Subdivisions of that group — . 

 Physical geography of the Oolite in England and France — Upper Oolite — Pur- 

 beck beds — New fossil Mammifer found at Swanage — Dirt-bed or ancient soil 

 — Fossils of the Purbeck beds-rPortland stone and fossils — Lithographic stone 

 of Solenhofen — Middle Oolite — Coral rag — Zoophj^tes — Nerinfean limestone — 

 Diceras limestone — Oxford clay, Ammonites and Belemnites — Lower Oolite, 

 Crinoideans — Great Oolite and Bradford clay — Stonosfield slate — Fossil mam- 

 malia, placental and marsupial — Resemblance to an Australian fauna — North- 

 amptonshire slates — Yorkshire Oolitic coal-field — Brora coal — ^Fuller's earth- — 

 Inferior Oolite and fossils. 



Immediately below the Hastings Sands (the inferior member of the 

 Wealden, as defined in the 1 8th chapter), we find in Dorsetshire another 

 remarkable freshwater formation, called the PurhecJc^ because it w^as first 

 studied in the sea-clifFs of the peninsula of Purbeck in Dorsetshire. 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 aflinity to the Oolitic group, of whicb 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 ooHtic structure (see 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. 



I! 



Purbeck beds. 

 Upper "I h. Portland stone and sand. 

 Kimmeridge clay, 



lyiiddle^^^^^^^^ft 



( e, Oxford clay. 



{/. Cornbrash and Forest marble. 

 g. Great Oolite and Stonesfield slato. 

 h. Fuller's earth. 

 i. Inferior Oolite. 



Tbe Lias then succeeds to the Inferior Oolite. 



