THE JURASSIC PERIOD. 22/ 



alternating with thin bands of blue or grey limestone — the 

 whole, when seen in quarries or cliffs from a little distance, 

 assuming a characteristically striped and banded appearance. 

 By means of particular species oi Anwionites^ taken along with 

 other fossils which are confined to particular zones, the Lower 

 Lias may be subdivided into several well-marked horizons. 

 The Middle Lias, or Ma?'lsione 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 whicli the following are the more 

 important groups : a, The Lnferior Oolite (Terraijt 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 ox Bath Oolite (Terj-ain Bath- 

 onien of D'Orbigny), consisting principally of oohtic 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-marble" 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 a great argillaceous de- 

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

 Clay (Terrai?t Calloviemxidi Terrain Oxfordien of D'Orbigny), 

 consisting of dark-coloured 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 Cor allien of D'Orbigny, 

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

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

 mass of oolitic lim.estone, 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 Kiinnieridge 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 



