Lias, in geology, the lowermost group of Jurassic strata. Originally the name seems to have been written “Lyas”; it is most probably a provincial form of “layers,” strata, employed by quarrymen in the west of England; it has been suggested, however, that the Fr. liais, Breton leach = a stone, Gaelic leac = flat stone, may have given rise to the English “Lias.” Liassic strata occupy an important position in England, where they crop out at Lyme Regis on the Dorsetshire coast and extend thence by Bath, along the western flank of the Cotswold Hills, forming Edge Hill and appearing at Banbury, Rugby, Melton, Grantham, Lincoln, to Redcar on the coast of Yorkshire. They occur also in Glamorganshire, Shropshire, near Carlisle, in Skye, Raasay (Pabba, Scalpa and Broadfoot beds), and elsewhere in the north of Scotland, and in the north-east of Ireland. East of the belt of outcrop indicated, the Lias is known to occur beneath the younger rocks for some distance farther east, but it is absent from beneath London, Reading, Ware, Harwich, Dover, and in the southern portion of the area in which these towns lie; the Liassic rocks are probably thinned out against a concealed ridge of more ancient rocks. The table on following page will serve to illustrate the general characters of the English Lias and the subdivisions adopted by the Geological Survey. By the side are shown the principal zonal ammonites, and, for comparison, the subdivisions preferred by Messrs Tate and Blake and by A. de Lapparent.
The important fact is clearly demonstrated in the table, that where the Lias is seen in contact with the Trias below or the Inferior Oolite above, there is, as a rule, a gradual passage from the Liassic formation, both downwards and upwards; hence Professor de Lapparent includes in his Liassique System the zone of Ammonites opalinus at the top, and the Rhaetic beds at the bottom (see Oolite; Rhaetic). Owing to the transgression of the Liassic sea the strata rest in places upon older Palaeozoic rocks. The thickness of the Lias varies considerably; in Dorsetshire it is 900 ft., near Bath it has thinned to 280 ft., and beneath Oxford it is further reduced. In north Gloucestershire it is 1360 ft., Northampton 760 ft., Rutland 800 ft., Lincolnshire 950 ft., and in Yorkshire about 500 ft.
The Lias of England was laid down in conditions very similar to those which obtained at the same time in north France and north Germany, that is to say, on the floor of a shallow sea; but in the Alpine region limestones are developed upon a much greater scale. Many of the limestones are red and crystalline marbles such as the “ammonitico-rosso-inferiore” of the Apennines; a grey, laminated limestone is known as the “Fleckenmergel.” The whitish “Hierlatzkalke,” the Adnet beds and the “Grestener beds” in the eastern Alps and Balkan Mountains are important phases of Alpine Lias. The Grestener beds contain a considerable amount of coal. The Lias of Spain and the Pyrenees contains much dolomitic limestone. This formation is widely spread in western Europe; besides the localities already cited it occurs in Swabia, the Rhenish provinces, Alsace-Lorraine, Luxemburg, Ardennes, Normandy, Austria-Hungary, the Balkan States, Greece and Scania. It has not been found north of Kharkov in Russia, but it is present in the south and in the Caucasus, in Anatolia, Persia and the Himalayas. It appears on the eastern side of Japan, in Borneo, Timor, New Caledonia and New Zealand (Bastion beds); in Algeria, Tunisia and elsewhere in North Africa, and on the west coast of Madagascar. In South America it is found in the Bolivian Andes, in Chile and Argentina; it appears also on the Pacific coast of North America.
| S. W. England and Midlands. | Yorkshire. | Ammonite Zones.* | Divisions according to A. de Lapparent.** |
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| Upper Lias. |
Midford Sands (passage beds) | Alum shale | Am. jurensis | U. | (Including the opalinus zone of the Inferior Oolite.) |
| Clays with Cement-stones | Jet Rock | Am. communis | Toarcien. | ||
| Limestones and Clays | Grey Shale | Am. serpentinus | |||
| Am. annulatus | M. | ||||
| Middle Lias. |
Marlstone and Sands (Rock Bed and Ironstones) |
Ironstone Series | Am. spinatus | Charmouthien. | |
| Micaceous Clays and Sands | Sandy Series | Am. margaritatus | |||
| Lower Lias. |
Clays with occasional bands of Limestone |
Upper Series with Ironstone nodules |
Am. capricornus Am. Jamesoni and Am. armatus |
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| Limestones and Clays | Lower Series with Sandy and Marly Beds |
Am. oxynotus Am. Bucklandi Am. angulatus Am. planorbis |
L. | Sinémourien. Hettangien including “White Lias.” |
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| Rhétien. | |||||
| * The brackets indicate the divisions made by R. Tate and J. F. Blake. | |||||
| ** Traité de géologie (5th ed., Paris, 1906). | |||||
The economic products of the Lias are of considerable importance. In the Lower Lias of Lincolnshire and the Middle Lias of Oxfordshire, Northamptonshire, Lincolnshire, Leicestershire and Yorkshire the beds of ironstone are of great value. Most of these ores are limestones that have been converted into iron carbonate with some admixture of silicates; they weather near the surface into hydrated peroxide. At Frodingham in Lincolnshire the oolitic iron ore reaches 30 ft. in thickness, of which 12 ft. are workable. In Gloucestershire the top beds of the Lower Lias and lower beds of the Middle division are the most ferruginous; the best ores near Woodstock and Banbury and between Market Harborough and Leicester are at the summit of the Middle Lias in the Marlstone or Rock bed. The ironstone of Fawler is sometimes known as Blenheim ore. The ores of the Cleveland district in Yorkshire have a great reputation; the main seam is 11 ft. thick at Eston, where it rests directly upon the Pecten Seam, the two together aggregating 15 ft. 6 in. Similar iron ores of this age are worked at Meurthe-et-Moselle, Villerupt, Marbache, Longuy, Champagneulles, &c. Some of the Liassic limestones are used as building stones, the more important ones being the Lower Lias Sutton stone of Glamorganshire and Middle Lias Hornton stone, the best of the Lias building stones, from Edge Hill. The limestones are often used for paving. The limestones of the Lower Lias are much used for the production of hydraulic cement and “Blue Lias” lime at Rugby, Barrow-on-Soar, Barnstone, Lyme Regis, Abertham and many other places. Roman cement has been made from the nodules in the Upper Lias of Yorkshire; alum is obtained from the same horizon. A considerable trade was formerly done in jet, the best quality being obtained from the “Serpentinus” beds, but “bastard” or soft jet is found in many of the other strata in the Yorkshire Lias. Both Lower and Upper Lias clays have been used in making bricks and tiles.
Fossils are abundant in the Lias; Lyme Regis, Shepton Mallet, Rugby, Robin Hood’s Bay, Ilminster, Whitby and Golden Cap near Charmouth are well-known localities. The saurian reptiles, Ichthyosaurus and Plesiosaurus, are found in excellent preservation along with the Pterodactyl. Among the fishes are Hybodus, Dapedius, Pholidophorus, Acrodus. The crinoids, Pentacrinus and Extracrinus are locally abundant. Insect remains are very abundant in certain beds. Many ammonites occur in this formation in addition to the forms used as zonal indexes mentioned in the table. Lima gigantea, Posidonomya Bronni, Inoceramus dubius, Gryphaea cymbium and G. arcuata are common pelecypods. Amberleya capitanea, Pleurotomaria anglica are Lias gasteropods. Leptaena, Spiriferina, Terebratella and Rhynchonella tetrahedra and R. variabilis are among the brachiopods.
Certain dark limestones with regular bedding which occur in the Carboniferous System are sometimes called “Black Lias” by quarrymen.
See “The Lias of England and Wales” (Yorkshire excepted), by H. B. Woodward, Geol. Survey Memoir (London, 1893); and, for Yorkshire, “The Jurassic Rocks of Britain,” vol. i., “Yorkshire,” by C. Fox-Strangways, Geol. Survey Memoir. See also Jurassic.
