I 718 GEOLOGY. 



The system, as developed in England, presents the following groups, each having its 

 subordinate divisions : 



"The Weald beds, stiff, blue, laminated clays, with shelly limestone beds, called Sussex or 

 Petworth marbles. 



Wealden 

 Group. 



The Hastings beds, consisting of sands and sandstones, with alternating clays, shales, and 



limestones. 



The Ashburnham beds, which include clays, scales, sandstones, and grey limestones. 

 The Purbeck beds, comprising shales and shelly limestones the latter called Purbeck marble. 

 1 Portland stone, a limestone of concretionary structure ; and Kimmeridge clay, of a blue colour, 



calcareous, with sandy concretions. 



Coral rag, a limestone composed of corals, with shells and echini ; and Oxford clay, a dark 

 Oolite tenacious clay, including beds of calcareous grit. 



Group. I orn b rasnj a coarse shelly limestone ; forest marble, sand, with concretions of arenaceous lime- 

 stone ; great oolite, Bath stone, yellow freestone and limestone ; Stonesfield slate ; fullers' 

 earth, with marls and clays ; coarse limestone, with ferruginous and concretionary sands. 

 : Beds of dark-coloured shale, with nodules and beds of limestone. 



Lias 

 Group. 



Indurated marls, calcareous, sandy, and ferruginous strata. 

 Limestones, laminated, with partings of clay. 



With the exception of the Wealden group a local deposit, lying principally in Kent 

 and Sussex the system occupies an extent of country, reaching, in a wavy course and 

 almost uninterrupted line, from the north-east coast of Yorkshire, above Whitby, to the 

 cliffs of Lyme Regis, in Dorsetshire. It runs parallel to the new red sandstone, with 

 many minute flexures and irregularities, acquires a considerable breadth in Yorkshire, 

 narrows to a point before leaving that county towards the confluence of the Humber and 

 the Trent, re-appears immediately on the south of the former river, proceeds through the 

 midland counties, where it attains its greatest expansion, again contracting and becoming 

 exceedingly involved and intricate as it approaches the southern coast. Among the 

 principal places included within its range are Newark, Grantham, Leicester, Northampton, 

 Banbury, Cheltenham, Oxford, and Bath. On its eastern edge are Lincoln, Peterborough, 

 Huntingdon, and Shaftesbury, and, on its western, Wells and Gloucester. There is a 

 remarkably insulated outlying patch of the lias part of the system between Whitchurch 

 and Wem in Shropshire ; another occurs in South Wales ; and several, less considerable 

 and not so insulated, appear on its south-western border. Identical rocks are very 

 abundant in France, intersecting that kingdom in bands, extending north and south 

 from the coast of Normandy to near the borders of the Mediterranean, and from the west 

 coast near Rochelle to the banks of the Rhine. The French and Swiss Jura mountains 

 consist of harder limestones of this formation, and hence the title terrain Jurassique 

 applied to it. The system is developed north of the Alps from the Rhine to Vienna, 

 and, on the south, many of the beautiful Italian lakes are embosomed in it. The 

 limestone ridges of Illyria, Dalmatia, Albania, and Greece belong to it ; it appears in the 

 range of the Atlas ; and the southern slopes of the Himalaya are argillaceous rocks 

 corresponding to the English lias. 



Lias group. The base of the oolitic system is its most extensive member, and the 

 most remarkable in its organic contents. It consists of thin strata of argillaceous 

 limestones associated with clays, forming a thick bed of a blue or white colour. The 

 blue lias contains a large proportion of lime, which hardens into a strong cement when 

 under water, for which reason it was used in the construction of the Eddystone Lighthouse. 

 The white variety takes a high polish, and is the stone sometimes employed in lithography. 

 The clays of the lias occur in the form of soft shales, impregnated with bitumen and 

 iron pyrites. They are susceptible of slow combustion, which has taken place spon- 

 taneously when in contact with a sufficient supply of moisture. About the middle of 

 the last century the cliffs of lias clay near Lyme in Dorsetshire ignited after heavy 



