2 34 HISTORICAL PALEONTOLOGY. 



The Middle Lias, or Marlstone Series (Terrain Liasien of 

 D'Orbigny), may reach a thickness of 200 feet, and consists of 

 sands, arenaceous marls, and argillaceous limestones, sometimes 

 with ferruginous beds. The Upper Lias (Terrain Toarcien of 

 D'Orbigny) attains a thickness of 300 feet, and consists princi- 

 pally of shales below, passing upwards into arenaceous strata. 



II. THE LOWER OOLITES. Above the Lias comes a com- 

 plex series of partly arenaceous and argillaceous, but prin- 

 cipally calcareous strata, of which the following are the more 

 important groups: a, the Inferior Oolite (Terrain Bajocien 

 of D'Orbigny), consisting of more than 200 feet of oolitic 

 limestones, sometimes more or less sandy; b, The Fuller's 

 Earth, a series of shales, clays, and marls, about 120 feet in 

 thickness; c, The Great Oolite or Bath Oolite (Terrain Bath- 

 omen of D'Orbigny), consisting principally of oolitic lime- 

 stones, and attaining a thickness of about 130 feet. The well- 

 known " Stonesfield Slates " belong to this horizon ; and the 

 locally developed " Bradford Clay, " " Cornbrash, " and " For- 

 est-marbte " may be regarded as constituting the summit of 

 this group. 



III. THE MIDDLE OOLITES. The central portion of the 

 Jurassic series of Britain is formed by great argillaceous de- 

 posit, capped by calcareous strata, as follows : a, The Oxford 

 Clay (Terrain Callovien and Terrain Oxfordien of D'Orbigny), 

 consisting of dark-colored laminated clays, sometimes reach- 

 ing a thickness of 700 feet, and in places having its lower por- 

 tion developed into a hard calcareous sandstone (" Kelloway 

 Rock"); b, The Coral-Rag (Terrain Corallien of D'Orbigny, 

 " Nerinean Limestone " of the Jura, " Diceras Limestone " of 

 the Alps), consisting, when typically developed, of a central 

 mass of oolitic limestone, underlaid and surmounted by cal- 

 careous grits. 



IV. THE UPPER OOLITES. a, The base of the Upper 

 Oolites of Britain is constituted by a great thickness (600 feet 

 or more) of laminated, sometimes carbonaceous or bituminous 

 clays, which are known as the Kimmeridge Clay (Terrain Kim- 

 meridgien of D'Orbigny) ; b, The Portland Beds (Terrain Port- 

 landien of D'Orbigny) succeed the Kimmeridge clay, and con- 

 sist inferiorly of sandy beds surmounted by oolitic limestones 

 (" Portland Stone"), the whole series attaining a thickness of 

 150 feet or more, and containing marine fossils; c, The Pur- 

 beck Beds are apparently peculiar to Great Britain, where they 

 form the summit of the entire Oolitic series, attaining a total 



