Page:Collier's New Encyclopedia v. 09.djvu/573

LEFT TRIASSIC SYSTEM 503 TRIASSIC SYSTEM ward Island, the Connecticut valley; the W. side of the Hudson river, and S. W. through Pennsylvania into Virginia; North Carolina. Strata believed to be of the same age cover Mride areas in the W. territories, extending from the E. borders of the Rocky Mountains into Alaska, British Columbia, and Califor- nia. 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 occur- rence. On the whole the American stra- ta above referred to resemble the Eng- lish type of the Trias. But on the Pa- cific slope, in Northern California and Mexico, the strata yield a plentiful ma- rine fauna and resemble the type of the Alpine Trias. Life of the Period. — The predominant plants were cycads (Pterophyllum, Za- mites, etc.), horsetails (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 inver- tebrate life) being derived from the Rhastic, the Muschelkalk, and the marine strata of the Alpine Trias. Foraminif- era, sponges, star corals, and echino- derms were tolerably numerous. One of the most beautiful fossils is the lily cncrinite (Encrinus liliiformis) of the muschelkalk. Among lamellibranchs myophoria, avicula, pecten, cardium were common forms. A number of palaeozoic genera of gasteropods {Loxonema, Mur- chisonia, etc.) appear, commingled with newer forms. The same is the case with the cephalopods, such old genera as Or- tkoceras, Cyrtoceras, and Gonialtites oc- curring along with Ceratites and other species of the great tribe of Ammonites. This remarkable association of palaeozoic 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 pal- ate teeth (Ceratodiis), Labyrinthodonts abounded. Lizard-like reptiles {telerpe- ton, hyperodapedon) were numerous, while crocodiles (stagonolepis) made their first appearance. The same is the case of 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. Swimming reptiles (Nothosaums) 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 remark- able for having yielded the earliest relics of mammalia. Physical Conditions. — The British Tri- assic 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 ma- terials were precipitated. This inland sea covered a large part of England, and extended N. into Southern Scotland and across what is now the area of the Irish Sea into the N. E. of Ireland. It is possible also that the same sea stretched into Northern France. Another but small- er lake is indicated by the red sandstones of Elgin. On the Continent during a large part of the Triassic period an inland sea ex- tended W. from the Thiiringerwald across the Vosges country into France, and stretched N. 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 Great Britain, but the thick muschelkalk with its numerous marine forms seems to indicate an influx of water from the open sea. Afterward, however, this connection was closed, and the subsequent accumulations point to increasing salinity, during which chemi- cal formations (gypsum, rock salt, etc.,) took place, while the marine fauna dis- appeared. Toward the close of the pe- riod, after the great inland lake had been largely silted up, a partial influx of the sea introduced a fauna compara- ble to that of the English Rhaetic. It seems highly probable that the lands sur- rounding the inland lakes of Central and Northwestern 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 Eng- land are made up of grains so complete- ly 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 Great Britain were sandy des- erts before the inland sea attained its greatest development. The Alpine Trias, which is mostly marine, shows that, while continental and lacustrine condi- tions obtained in central and N. W. Eu- rope, an open sea existed toward the S. — 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