Lias

Lias, said to be so named from a provincial pronunciation of "layers," from its well-marked stratification, is the lowermost division of the English Jurassic system (q.v.). It extends continuously from Dorsetshire to the mouth of the Tees, outlying patches occurring at Carlisle, in Skye and the west of Scotland, and under the Chalk escarpment in the north of Ireland. It consists of three divisions: the Lower, dark shales, with thin blue and brown nodular bands of limestone; the Middle or Marlstone, argillaceous limestone, with micaceous sands and clays, and the "black band ironstone," 15 to 20 feet thick, of the Cleveland district in Yorkshire; and the Upper, blue clays and shales, with septarian nodules. It passes, by sandy passage-beds, conformably into the beds both above it and below. Its fossil plants comprise Equisetites, ferns, cycads, and conifers ( Cupressus, Pinites, etc.), whilst numerous insect-remains, including wood-boring beetles, dragon-flies, and others, also point to the proximity of land. The shallow-water marine life is abundantly preserved, including corals; such crinoids as Extracrinus; starfish; sea-urchins; numerous brachiopods, especially Rhynckcmella and Spiriferina, the last of its family; still more pelecypods, especially scallops (Pecten), Lima and Gryphcea; and many gastropods, such as Cerithium, Turbo, Trochus, and Pleurotomaria. The most characteristic molluscs, however, are the cephalopods, including species of Nautilus, 60 species of Belemnites, and upwards of 130 of the Ammonitidce. Many of these last have a small vertical range, but occur on the same relative horizons over most of western Europe, so that the whole formation is subdivided into zones (q.v.) named after the various species of this group. Fish are numerous, including the teeth (Acrodus, Ceratodus) and spines of placoids and whole ganoids, such as Lepidotus; but the large reptiles, such as Ichthyosaurus and Plesiosaurus, are so characteristic that the period has been termed the "age of reptiles." The Lias, or Brown Jura, of Germany much resembles that of England. Its upper member yields paraffin shale. In France and Switzerland the Lias is subdivided into four : Toarcien = Upper Lias.

Liassien = Middle Lias. Sinmurien = Lower Lias. Hettangian = Infra-Lias.