Triassic System

Triassic System, the lowest of the three great systems into which Secondary Rocks (q.v.) are divided, so named from its divisibility into three in the Harz Mountain area, where it was first described. It can generally, however, be better divided into four parts, known (in downward succession) as the Rhaetic beds, the Keuper Marls, the Muschelkalk, and the Bunter-Sandstein each of which is described separately. The Trias is essentially a great series of sandstone, the salt-lake origin of which, at least in its upper portions, is indicated by associated beds of rock-salt and gypsum. Resting commonly horizontally over the folded Carboniferous rocks, it was formerly classed with the Permian (q.v.) as the New Red Sandstone, but in Germany, though less markedly in England, the two are separated by unconformability, and there is a decided difference in the character of their fossils. In England the outcrop of the Trias forms an interrupted band from Exmouth, along the Severn valley towards Derby, where it is divided by the Pennine anticlinal, one half extending westward over Staffordshire and Cheshire to the mouth of the Ribble, and the other eastward through Nottingham and Yorkshire to that of the Tees. Deep borings show the Trias to thin-out in a south-eastern direction. The copper of the Keuper, rock-salt, as at Droitwich, and gypsum, are its chief economic products; but the coal-beds of Virginia and Carolina belong to this period, and coal is often worked from beneath it, as in the Lancashire and Cheshire coalfield. It is one of the best water-bearing series in England, and yields also some very valuable agricultural land. Fossils are mainly confined to the Muschelkalk and Rhaetic. Equisetites is the most characteristic plant, and Ceratites nodosus, the most characteristic cephalopod of the period, in which a certain mingling of Palaeozoic and Mesozoic molluscs is noticeable. The amphibian Labyrinthodon, the crocodilian Staqonolepis, and other reptiles (especially in the Elgin sandstone), footprints possibly avian, and the minute marsupial teeth that form the earliest indication of mammalian life, are the chief other remains of importance.