280 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



Triassic. — When we examine into the Triassic faunas we meet at once with a wholly new 

 marine assemblage. The late Paleozoic world of fusulinids, tetracorals, crinids, brachi- 

 opods, nautilids, and trilobites had either vanished or was represented by a few small and 

 rare forms. On the other side, in the Triassic, their places were taken by a rising marine 

 world of small invertebrates, now hexacorals, regular echinids, modern bivalves (among 

 them the oysters), siphonate gastropods, and more especially by a host of ammonites and a 

 prophecy of the coming of squids and marine reptiles. Truly, there is no greater change 

 recorded in all Historical Geology! 



Plants are scarce in the rocks of Triassic time until near its close in the Rhactic, when 

 we can again truly speak of Triassic floras. These are known from many parts of the 

 world, and according to Knowlton there is nothing in the floras to suggest a "depau- 

 Ijerate and pinched" condition, as has often been said. "In North Carolina, Virginia 

 and Arizona, there are trunks of trees preserved, some of which are 8 feet in diameter 

 and at least 120 feet long, while hundreds are from 2 to 4 feet in diameter. Many of the 

 ferns [some are tree ferns] are of large size, indicating luxuriant growth, while Equisetum 

 stems 4 to 5 inches in diameter are only approached by a single living South American 

 species. * * * The complete, or nearly complete absence of rings in the tree trunks 

 indicates that there were no, or but slight, seasonal changes due to alterations of hot and 

 cold, or wet and dry periods." On the whole, the climate was "warm, probably at least 

 subtropical" (1910a: 200-2). 



Of insects, too few species (27) are known to be of value for climatic deductions. On 

 the other hand, the reptilian life of the Triassic in America, Africa, and Europe was highly 

 varied, and with the dinosaurs dominant and often of large size again gives evidence that 

 appears to be indicative of uniform and mild climate. 



The marine Triassic deposits consisted largely of thick limestones, and such are well 

 dovelojDed in Arctic America and Ai'ctic Siberia. One of the oldest faunas, known as the 

 Meekoceras fauna, has a very great distribution from Spitzbergen to India and Madagascar, 

 and from Siberia at Vladivostok to CaHfornia and Idaho. In general, howe^'er, the Tri- 

 assic assemblages were more provincial, and it was not until middle and late Triassic time 

 that the faunas again had wide distribution. Limestones with thick coral reefs, of the 

 same age, appear in the Alps (up to 1,000 meters thick), India, California, Nevada, Oregon, 

 and Arctic Alaska. Smith, from whom most of these facts were taken, states that this 

 shows there was during the Triassic "nearly uniform distribution of warm water over a 

 great part of the globe" (1912a: 397-8). 



We may therefore conclude that the rigid climate of the Permic had vanished even 

 before the earliest of Triassic times, and that the climate of the latter period until near its 

 close was again mild and fairly uniform though semiarid or even arid the world over. 



Late Triassic-Lias. — Throughout much of late Triassic time there was renewed crustal 

 instability, for we have the evidence of volcanism on a great scale all along the Pacific from 

 central California into far Alaska, in eastern North America from Nova Scotia to Virginia, 

 in Mexico, South America (in southern Brazil 600 meters thick), and New Zealand. The 

 volcanoes of western North America were probably insular in position, for their lavas and ash 

 beds are found interbedded with marine sediments. Just how important this movement 

 was and what effect it had upon the climate is not yet clear, but there is im^portant organic 

 evidence leading to the belief that the temperature was considerably reduced during latest 

 Triassic and earliest Jurassic time. 



Pompeckj, Buckman, and Smith state that late Triassic time was a particularly critical 

 one for the ammonites. Of the far more than 1,000 known species of Triassic ammonites, 

 not one passed over into the Jurassic, and but a single family survived this time, the Phyl- 

 loceratidse. Pompeckj says that "out of Phylloceras has developed the abundance of 

 Jurassic-Cretaceous ammonites" (1910: 64), while Buckman holds it was out of Nannites 

 by way of the Liassic Cymbites that the later fullness of ammonite development came. 



