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 of Ammonites, 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 Marlstone Scries (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 Toarden 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 Bajocicn 

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

 limestones, sometimes more or less sandy ; b, The fullers 

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

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

 onien 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-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 (Terrain Callovien and 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 Corallicn 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 ieet 

 or more) of laminated, sometirces 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 



