SHE, JURASSIC PERIOD. 227 
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 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 Oo.ires.—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: @, The Juferior Oolite (Terrain Bazjocien 
of D’Orbigny), consisting of more than 200 feet of oolitic 
limestones, sometimes more or less sandy; 4, The /udler’s 
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. 
Ill. THe Mippie Oo.ires.—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”); 4, The Coral-Rag (Zerrain 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 Oo.ites.—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 Avmmeridge Clay (Terrain Kim- — 
méridgien of D’Orbigny); 6, The Portland Beds (Terrain Port- 
landien of D’Orbigny) succeed the Kimmeridge clay, and con- 
sist inferiorly of sandy beds surmounted by oolitic limestones 
