628
THEORY AND PRACTICE OF ENGINEERING.
Book If.
were in a softer state than they are now found, many of the roots having penetrated,leaving cavities resembling them in shape. In all probability the Oolitic beds which wereformed in the sea first became dry land, were then covered by a forest, and afterwardssubmerged beneath a body of fresh water, from the sediment of which fiuviatile shellswere deposited.
The cap of the Portland Oolite is light-coloured, and of a fine grain, but shattered andfull of fissures, which occasions it to fall into masses too small for any useful purpose, althoughit is not destitute of hardness.
The skull cap is compact and open in its texture, hard, and its grain well cementedtogether; it is useful for structures where ornament is not required. The Roche ofboth the top and bottom beds forms a portion of the stone with which it is in contact, andresembles it in its composition. The best beds are those called the top, which are situatedabout midway between the top and bottom of the quarry. Their thickness varies from 5to 7 feet, although occasionally blocks of much greater dimensions are obtained. Thebottom bed averages about 6 feet in depth, but the particles are not so well cementedtogether. They are in a loose state of combination ; it is, however, often preferred, fromthe facility with which it may be worked, and from its fine grain.
Kimmeridge clay is of a blue colour, and consists in a great part of bituminous shale,sometimes forming an impure coal, several hundred feet in thickness, whilst in others itresembles peat. The impressions of plants are rarely found in it, but Ammonites, Ostreadeltoida, Gryphaea virgula, are mixed with the shale, and the bitumen it contains may beattributable to their animal origin.
Middle Oolite. — Upper Calcareous grit is a deposite full of comminuted shells, having anoblique lamination ; it is a freestone of rather close texture.
Coral rag, or Coralline Oolite consists of two beds of calcareous sand separated byanother ot limestone ; the lowest is of a yellowish colour, and contains about thirty per cent,of calcareous matter. The limestone is almost wholly composed of corals. The upperarenaceous deposit is close in its texture, full of comminuted shells, forming a calcareousfreestone, which is useful for many purposes. Among the organic remains are the Am-monites vcrtebralis, Plagiostoma rigidum, Clypeus dimidiatus, Cidaris florigemma, Astreatabulifera, and Caryophylla annulata, and frequently masses of coral as they were formedin the sea.
Lower calcareous grit is a yellowish quartzose sand, containing a considerable quantity ofcalcareous matter.
Oxford clay is an argillaceous deposit of a dark blue colour, which, when exposed tothe action of the air, becomes brown ; it contains Septaria, and concretions of argillaceouslimestone separated by veins of calcareous spar ; among the organic remains is theGryptuea dilatata.
Kelloway rock is a calcareous sandstone abounding in fossils, among which is the Ammo-nites calloviensis.
Lower Oolite. —500 feet in thickness in the neighbourhood of Bath.
Combrash limestone is a coarse and impure limestone, though it has considerable hardness,and is of a compact nature. It has sometimes a crystalline appearance, and contains manyshells, &c.
Forest marble is found associated with clay and sand ; it is limestone of a bluish tint,with particles of oolite occurring in thin beds. In Wichwood Forest in Oxfordshire , itbecomes a coarse kind of marble, which is worked for inferior purposes.
Bath Oolite occurs in beds of from 100 to 200 feet in thickness, and is calcareous; whenfreed from fossils it is easily sawn and worked with the chisel ; ii contains various eoraJs,as the Eunomia radiata, several of which have been found of great dimensions, indicating agrowth of many centuries, as the large brain coral, the Meandrina. The stone lilies(Crinoideans) are also common. The upper surface of the Bath or Great Oolite onceabounded with a forest of these beautiful zoophytes ; and there is every appearance thatthe deep marine waters in which they were produced were invaded by deposites of mud,which threw down and destroyed them; the stumps of some are found in their originalposition. Among the other fossils are the Trigonia gibbosa, Ostrea Marshii, Orbiculareflexa, Ammonites striatulus, &c.
Stonesfield slate is remarkable for its marine and terrestrial remains : it is calcareous,and consists of two beds of thinly laminated oolitic limestone, of about 2 feet in thickness,separated by calcareous limestone; it has the quality of splitting pr separating intoplates when struck with a mallet; after being exposed during a winter it is occasionallyused for roofing. It is obtained from the quarries by driving a gallery into the side of thehill, about 6 feet in height, and as it is exhausted the sandstone is piled or built up tosupport the superincumbent earth. The Stonesfield slate is rich in organic remains,containing Belemnites, Trigonia*, and other marine shells, many impressions of ferns,Cycadea?, and other terrestrial plants, several insects, and many genera of reptiles, asthe Plesiosaurus, Crocodile, and Pterodactyl, and the jaws of two mammiferous quadrupedsallied to the Opossum (Didelphys).