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The White Lias comprises many beds of white limestone, capped by a hard smooth-grained stone called the Sun bed, which from its closeness of texture and general purity has been recommended for purposes of lithography. At or near the base of the White Lias is found the Cotham or Landscape Marble, so well known from the dendritic markings which occur throughout it.' It is found in impersistent masses from two or three to eight inches in thickness, and is present generally where the White Lias is developed in the southern part of Gloucestershire, in Somersetshire, even to the coast-section near Lyme Regis in Dorsetshire.
The black-shales or paper-shales form perhaps the most. characteristic and persistent portion of the Rhætic series. They are much impregnated with iron pyrites, and contain the well-known bone-bed,2 of which sometimes there are one or two bands: these consist of pyritic and micaceous sandstone containing the bones, teeth, palates and coprolites of fish.
The Grey Marls at the base of the Rhætic series graduate so imperceptibly into the Red Marls of the Trias, that sometimes they are included in that formation. The mere difference in colour between the deposits is subject to so much variation that although the red disappears in the Rhætic marls, yet the boundary-line drawn solely in reference to colour can never be taken as a true physical horizon.
The Rhætic beds, where they approach the old Palæozoic land of the Bristol Coal Basin, the Mendip Hills or South
1 It is considered that these markings are produced by infiltration of ores of manganese and iron.
2 Formerly called the Lias Bone-bed.
Wales, frequently overlap the New Red rocks, whether Red Marl or Dolomitic Conglomerate, and repose directly upon the older rocks. They then sometimes present conglomeratic characters. (See Fig. 8, p. 84.)
The Rhætic beds, wherever sections have been obtained, have been found to be present between the Red Marls and Lower Lias. They may therefore be said to extend across England from near Redcar on the coast of Yorkshire to near Lyme Regis on the coast of Dorsetshire.
They occur in Cumberland near Carlisle, in Staffordshire (Needwood Forest), Shropshire, Warwickshire, Leicestershire, and in Glamorganshire, South Wales.
In Yorkshire 10 to 20 feet of blue or grey limestone, generally laminated and shelly, with partings of whitish clay or marl, rests immediately upon the red gypseous marls of the Keuper.
The White Lias seems to be feebly represented in Lincolnshire, Nottinghamshire, and Leicestershire, and there appears to be a development of greyish marly beds in part replacing the White Lias, which becomes so conspicuous in Somersetshire and Dorsetshire. At Penarth the White Lias
is scarcely traceable.
The Rhætic beds are well exposed near Watchet, where the thickness is 150 feet, which is only equalled elsewhere in England at Queen Camel near Yeovil.
Amongst other sections are those of Garden Cliff, Westbury-on-Severn (Westbury Beds), Aust Cliff, Saltford, Uphill cutting, Puriton and Shepton Mallet.
The Wedmore stone is a shelly limestone locally met with in the Rhætic beds near Wedmore; and the Flinty bed' is a compact limestone with alternating shelly layers, found near Beer Crocombe, in which Mr. Moore has obtained many species of mollusca.
The fossils of the White Lias are Ostrea Liassica, Mo
diola minima, Cardium Rhæticum, Monotis decussata, Annelid-tubes, &c.
In the Black-shales and intercalated bone-beds and thin sandy seams are Avicula contorta; Pecten Valoniensis, &c.
FIG. 14.-Section of Lower Lias and White Lias, near Bath.
(Midland Railway, near Weston Station, looking east.)
Sandy beds are sometimes found, crowded with casts of Pullastra arenicola.
The grey marls have yielded but one fossil, but that is of the highest interest, being the tooth of the oldest known British Mammal, Microlestes (Hypsiprimnopsis) Rhæticus, which was found by Mr. Boyd Dawkins at Watchet.
Edward Forbes (nearly 30 years ago) expressed his opinion that the fauna of the White Lias was curiously representative of the existing Caspian fauna. He then broached the notion that the Red Marls were formed in a great salt inland sea (a sort of Aralo-Caspian), during the last state of which the White Lias was formed; that the bed was then (if not 1 Hence the beds have been termed the zone of Avicula contorta.
elevated) depressed and turned into a part of the ocean, when the Liassic fauna came in.1
At a meeting of the British Association held at Bath in 1864, Mr. Moore described three cartloads of deposit containing Rhætic fossils which he had found washed into a fissure of Mountain Limestone near Frome. From this he had obtained twenty-nine teeth of the oldest Mammals (Microlestes Moorei), three only having been previously found, together with relics of nine genera of Reptiles, most of them new to this country, and fifteen genera of Fishes. Mr. Moore also produced 70,000 teeth of the Lophodus alone as the result of his labour, and stated that the three loads of clay had yielded him probably one million specimens.
The White Lias beds are used for building purposes, and are burnt for lime. The grey marls have been used for marling land.
The Rhætic beds generally form a gentle escarpment overlooking the vales of New Red Marl, and the junction with the beds below is frequently conspicuous in the ploughed fields. Springs are given out at the base of the White Lias.
1 Memoir of Edward Forbes, 1861, p. 418; see also Ramsay, Quart. Journ. Geol. Soc., vol. xxvii. p. 189.
THE Jurassic system includes the several members of both Lias and Oolites, and derives its name from the development of these strata in the Jura mountains between France and Switzerland,
The Liassic and Oolitic rocks have been conveniently grouped into different stages or sections, which are characterized by certain marked lithological features, and by certain assemblages of organic remains. Although these characters are remarkably persistent over large areas, yet the different stages display a considerable variation in thickness when traced across the country, and some of them are quite locally developed.
When we proceed to correlate the Liassic and Oolitic rocks of the south-west of England with the same beds in the midland and north-eastern counties, we find that many difficulties present themselves. The variations in thickness, in the lithological subdivisions, and in the assemblages or zones of organic remains, are striking. When, however, we look to the physical history of the period, and the method of deposition of the sediments, such changes appear but natural. We cannot for one moment suppose that an argillaceous, a sandy, or a calcareous deposit was formed