516 F. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



The Lepidodendrse, Sigillariae, Calamites, Cordaites, Sphaenophyllte, and 

 Cycadofilices, so far as ascertained, have all disappeared, as well as a 

 number of important genera of ferns — Clieilanthes, Mariopteris, Mega- 

 lopteriSj etcetera. The most notable survival of the Paleozoic flora is 

 the so-called Glossopteris flora, which has been found with a few asso- 

 ciated forms in Ehsetic rocks at Tonkin, the Stromberg series of South 

 Africa, in New South Wales, etcetera. 



The Triassic flora consists essentially of ferns, equisetums, cycads, 

 and conifers of many genera. A few forms, such as Ginkgo, Cladoph- 

 lepisj Thinnfeldia, etcetera, had a small beginning in the Paleozoic and 

 expanded in the Mesozoic into large groups. 



It has often been said that the plants of the Triassic are depauperate 

 and pinched in aspect, which is interpreted as indicating unfavorable 

 environmental or climatic conditions, but the paleobotanical facts do not 

 altogether bear out this contention. The earlier Triassic rocks are 

 largely composed of so-called red beds that are interpreted by many 

 geologists as an indication that they were laid down under condition^j 

 of aridity. As already pointed out, this view has been brought mora 

 or less in question of late, but whatever the result of the discussion the 

 fact remains that they are largely barren of fossils, and hence our knowl- 

 edge of the flora — especially the upland flora — of early Triassic time 

 is limited. In the Upper Triassic rocks, however, there is every indica- 

 tion that climatic conditions were not unfavorable to vigorous plant 

 growth. Thus in Xorth 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 are of very large size, indicating luxuriant growth, 

 while Equisteum stems 4 to 5 inches in diameter are only approached by 

 a single living South American species. The cycads and conifers are 

 not more depauperate than those of subsequent horizons, nor do they 

 compare unfavorably with the living representatives. The complete, or 

 nearly complete, absence of gTowth rings in the tree-trunks indicate that 

 there were no — or but slight — seasonal changes due to alternations of 

 hot and cold or wet and dry periods. The accumulation of coal — in the 

 Virginia area aggregating 30 to 40 feet in thickness — indicates long- 

 continued swamp or marsh conditions, while the presence of tree-ferns 

 seems to indicate, on the whole, a moist, warm, probably at least sub- 

 tropical, climate. 



Late Triassic and early Jurassic time. — During late Triassic time 

 there was renewed crustal movement, as evidenced by volcanic activity 



