TRIASSIC SYSTEM or TRIAS, geological terms used to designate the lowest major division of the Mesozoic era—given by F. von Alberti (1834) owing to the division of the system into a threefold series in Germany, in contradistinction to the twofold division (Dyas) of the underlying Permian.
Like the Permian (q.v.) the Trias is represented by two phases of sedimentation, the one continental as in Germany, the other marine—the latter being the normal aspect.
Where the marine facies is developed the division between the Permian and Triassic systems, or in other words between the Palaeozoic and Mesozoic eras, can best be drawn by means of the faunas. Where the continental facies occurs fossil evidence is scanty, and the general similarity of the deposits, together with their rapid changes in lithological type, makes it difficult to define the boundaries of the two systems. In Britain they are so closely knit that the term New Red Sandstone was formerly suggested by Goodchild for the combined Permo-Trias, and there is a tendency to revert to that grouping.
Records of the continental phase reside in breccias, con glomerates, red and mottled sandstones, marls and clays, with beds of dolomite, limestone and coal, and layers of gypsum, anhydrite or rock-salt. The coarser deposits consist of frost- or sun-riven scree material accumulated almost in situ, or as wide spreading torrential fans. Conglomerates afford evidence of trans portation by water, whilst the oblique bedding of certain sand stones, coupled with the perfect rounding of many of the grains and the dreikanter form of some of the enclosed pebbles, point to the action of wind. Some of the sands may represent dunes, but the majority were deposited with layers of mud in sheets of water which were shallow and subject to frequent desiccation. Of this we have evidence in reptilian footprints, fossil rain-pits, ripple-marks, sun-cracks and precipitated salts. Fresh influxes of water are shown by pellets of sun-dried mud and salt-pseu domorphs included in the sandstones.
In North and Central Germany the Zechstein clays are suc ceeded conformably by clayey Bunter beds. In other districts there is unconformity, with overlap of higher members of the Bunter.
The Roth contains rock-salt and bivalve-bearing beds of lime stone and dolomite (Rhizocorallum Dolomite).
The Muschelkalk—mainly calcareous—is the only division with a considerable fauna, which is poor, however, in comparison with that of the marine deposits of the Alps. In extent it is inferior to the Bunter, its westward limits stopping short of central France, although it occurs near Toulon and Montpellier. In Alsace-Lorraine the Lower Muschelkalk takes on a sandy facies (Muschelsandstein) as does the Middle division in part of Luxembourg. The Upper Muschelkalk is the richest in fossils, being characterized by Ceratites nodosus, C. semipartitus and C. enodis.
The Keuper (Marnes irisees of France) comprises red and variegated clays, pale sandstones, limestones, dolomites and im pure coals. Estuarine conditions in the Kohlenkeuper are shown by Myophoria goldfussi and Esthericz minuta as well as relics of fishes (Acrodus, Ceratodus, etc.), and terres trial conditions by Labyrinthodonts and Saurians (Mastodon saurus, etc.). The Gypsekeuper contains rock-salt in Lorraine. Stubensandstein and Schlifsandstein occur in south Germany. The former yields saurian remains (Aetosaurus ferratus and Belodon kappfi), the latter plants (Equisetum arenaceum, etc.).
Rhaetic beds, typical of the Rhaetic Alps, mark the return into western Europe, by a wide-spread marine transgression, of the seas that characterized the subsequent Mesozoic era, and form a passage between Keuper and Lias (Infra-Lias of France), although the molluscan fauna and the flora suggest a Triassic age. In Germany the Rhaetic consists mainly of pale sandstones and grey shales. The fauna—with Avicula (Pteria) contorts, Protocardium rhaeticum, and other forms—is not rich, but highly important, because of its great extent. Remarkable features are the bone-beds, a few inches thick, which occur at several horizons, crowded with teeth, bones, scales and coprolites of fishes and reptiles. Here are found Ichthyosaurus and Plesiosaurus, anticipating their maximum development; while remains of Belodon and Mystriosuchus serve as a link with Triassic Stegocephalian reptiles. Teeth of Microlestes antiquus, the oldest known mammal, occurred near Stuttgart.