Lias. The lias is the lowest division of the Jurassic System (q.v.). The beds composing it may be considered as the argillaceous basis of that series of rocks, consisting of more than a thousand feet of alternations of clay and limestone, with but a few unimportant deposits of sand. It consists of the following groups: Upper Lias (400 feet), Marlstone (200), Lower Lias (900).
The Upper Lias consists of thin limestone beds scattered through a great thickness of blue clay, more or less indurated, and so aluminous that it has been wrought for alum at Whitby. Above this clay come sandy deposits. The Marlstone is an arenaceous deposit, bound together either by a calcareous or ferruginous cement, in the one case passing into a coarse shelly limestone, and in the other into an ironstone, which has been extensively wrought both in the north and south of England. The Lower Lias beds consist of an extensive thickness of blue clays, intermingled with layers of argillaceous limestone. In weathering, the thin beds of blue or gray limestone become light brown; while the inter-stratified shales retain their dark colour, giving the quarries of this rock at a distance a striped or ribbon-like appearance, whence it is supposed the miner's name lias or layers is derived.
The Lias is highly fossiliferous, the contained organisms being well preserved; the fishes are often so perfect as to exhibit the complete form of the animal, with the fins and scales in their natural position. Numerous remains of plants occur in the lignite and in the shales. The name Gryphite Limestone has been given to the Lias, from the great quantities of Gryphea incurva, a kind of oyster, found in it. Fish-remains are frequently met with; the reptiles, however, are the most striking forms. They are remarkable for the great numbers in which they occur, for the size which many of the species attain, and for the adaptations in their structure which fitted them to live in water. The most noteworthy are species of Ichthyosaurus and Plesiosaurus (q.v.).
The Liassic rocks extend in a belt of varying breadth across England, from Whitby, on the coast of Yorkshire, south to Leicester, then south-west by Gloucester to Lyme Regis in Dorsetshire.