Triassic System.

Triassic System. This forms the basement group of the Mesozoic or Secondary strata, and was formerly associated with the Permian System under the name of the New Red Sandstone. The term trias has reference to the threefold grouping of the system in Germany, where the strata are more fully developed than in Britain. In our area the system rests unconformably upon the upturned and denuded edges of the Permian and older Palaeozoic strata. It is well developed in the central plains of England, whence a long belt extends north from Nottingham to the valley of the Tees, while another band stretches down the Severn valley into Devonshire. Small areas likewise occur in Dumfriesshire and near Elgin, and also in the north of Ireland. The system, however, assumes more importance in central Europe, where it occurs at the surface over a wide tract between the Thüringerwald in the east and the Vosges Mountains in the west, and between Basel in the south and Hanover in the north. North of that region it continues underneath overlying formations, but appears again and again at the surface where these latter are wanting. Trias is also met with in Heligoland and the south of Sweden. In all the regions now noted the strata appear to have been deposited in inland seas, and the following table gives the general succession of strata:

In the Alpine regions the Trias differs much from that of England and Germany. It attains a thickness of many thousand feet, and forms ranges of mountains. The lower division consists chiefly of fossiliferous limestones, the middle of shales, marls, limestones, and dolomites, while the Rhætis is built up mainly of limestones and dolomites. Thus in north-western and central Europe we have one well-defined type consisting of strata which have accumulated for the most part in inland seas, while in the Alpine regions the character of the beds betokens more open water. In France isolated areas of Trias occur, some of which approximate in appearance to those of England, while others resemble those of Germany. In Spain and Portugal both the German and Alpine types are represented.

In North America the Trias is well developed, as in Nova Scotia, Prince Edward Island, the Connecticut valley; the west side of the Hudson River, and south-west through Pennsylvania into Virginia; North Carolina. Strata believed to be of the same age cover wide areas in the western territories, extending from the eastern borders of the Rocky Mountains into Alaska, British Columbia, and California. Brick-red sandstones and marls are a prominent feature in all those areas. Like the similar rocks of Europe they contain few fossils, but animal tracks and footprints are of frequent occurrence. On the whole the American strata above referred to resemble the English type of the Trias. But on the Pacific slope, in Northern California and Mexico, the strata yield a plentiful marine fauna, and resemble the type of the Alpine Trias. Rocks of Triassic age have been recognised in Spitzbergen, in the Himalayas, and again in South Africa, in New South Wales, Victoria, and Queensland, where the series contains coal-seams.

Life of the Period.—The predominant plants were cycads (Pterophyllum, Zamites, &c.), horse-tails (Equiseta), ferns, and conifers, especially the cypress-like Voltzia. In the red beds of the Trias few fossils occur, our knowledge of the life of the period (more especially the invertebrate life) being derived from the Rhætic, the Muschelkalk, and the marine strata of the Alpine Trias. Foraminifera, sponges, star-corals, and echinoderms were tolerably numerous. One of the most beautiful fossils is the lily encrinite (Encrinus liliformis) of the Muschelkalk. Amongst Lamellibranchs Myophoria, Avicula, Pecten, Cardium were common forms. A number of Palæozoic genera of Gasteropods (Loxonema, Murchisonia, &c.) appear, commingled with newer forms. The same is the case with the Cephalopods, such old genera as Orthoceras, Cyrtoceras, and Goniatites occurring along with Ceratites and other species of the great tribe of Ammonites. This remarkable association of Palæozoic and Mesozoic genera is most notable in the Alpine Trias. In the same strata occur the earliest traces of dibranchiate cephalopods, represented by the internal bone or shell (Belemnites). The Triassic fishes are ganoids and placoids—the latter represented by spines and palate teeth (Ceratodus). Labyrinthodonts abounded, and are known chiefly through their footprints, which are often plentiful in the red beds of the Trias. Some of these creatures attained a large size—the skull of one (Mastodonsaurus) measuring over 3 feet in length by 2 feet in breadth. Lizard-like reptiles (Telertpeton, Hyperodapedon) were numerous, while crocodiles (Stagonolepis) made their first appearance. The same is the case with the extinct group of Dinosaurs—terrestrial reptiles, some of which could walk on their hind-feet, which were often only three-toed—their front feet being four-toed. The footprints of these Dinosaurs are very numerous in some sandstones, and the three-toed impressions were at first supposed to be those of birds. They vary in size, the largest being nearly 2 feet long. Swimming reptiles (Nothosaurus) have also been recorded from the Trias. Another remarkable group of reptiles were represented by Dicynodon, which had a horny beak and carried two large tusk-like teeth in the upper jaw. The Trias is further remarkable for having yielded the earliest relics of mammalia. They seem to have been small marsupials (Microlestes, Dromatherium), with some affinities to the little Banded Ant-eater of Australia.

Physical Conditions.—The British Triassic strata afford evidence of having, for the most part, been deposited in a great inland sea or salt lake, from the waters of which sodium chloride (rock-salt), gypsum, and other chemically-formed materials were precipitated. This inland sea covered a large part of England, and extended north into southern Scotland and across what is now the area of the Irish Sea into the north-east of Ireland. It is possible also that the same sea stretched into northern France. Another but smaller lake is indicated by the red sandstones of Elgin. The lands surrounding these lakes were clothed with cypress-like evergreens, and their shores were haunted by labyrinthodonts and various reptiles. The briny waters were unfavourable to life, and we have consequently few traces of any aquatic fauna, which seems to have consisted chiefly of small phyllopods (Estheria) and fishes. Eventually the laeustrine areas became largely silted up, and then subsidence of the land took place, so that the sea occupied some of the shallow depressions. In these marine tracts the Rhætic beds were deposited.

On the Continent during a large part of the Triassic period an inland sea extended westwards from the Thüringerwald across the Vosges country into France, and stretched northwards from the confines of Switzerland over what are now the low grounds of Holland and North Germany. In this ancient sea the Harz Mountains formed an island. In the earlier stages of the period the conditions resembled those that obtained in Britain, but the thick Muschelkalk with its numerous marine forms seems to indicate an influx of water from the open sea. Afterwards, however, this connection was closed, and the subsequent accumulations point to increasing salinity, during which chemical formations (gypsum, rock-salt, &c.) took place, while the marine fauna disappeared. Towards the close of the period, after the great inland lake had been largely silted up, a partial influx of the sea introduced a fauna comparable to that of the English Rhætic. It seems highly probable that the lands surrounding the inland lakes of central and north-western Europe were more or less dry and sandy regions, like the great wastes of central Asia. Many of the sandstones in the Bunter series of England are made up of grains so completely worn and rounded that they exactly recall the appearance presented by the wind-blown sands of desert regions. Some geologists therefore infer that in the earlier stages of the Triassic period large tracts of Britain were sandy deserts before the inland sea attained its greatest development.

The Alpine Trias, which is mostly marine, shows that, while continental and lacustrine conditions obtained in central and north-western Europe, an open sea existed towards the south—a Mediterranean of much greater extent than the present. From the fact that Triassic rocks with characteristic fossils occur within the Arctic regions, it may be inferred that the climate of the period was generally genial or warm.