512 F. H. KNOWLTOX EVOLUTIO>' OF GEOLOGIC CLIMATES 



there came a remarkable change in the flora, which was perhaps due to — 

 or in any event was accompanied by — renewed sea expansion. Many 

 new types were either introduced or became dominant element*, such 

 as Cheilanthes (Pseudopecopteris in part), Eremopteris, Lonchopteris, 

 Megalopteris. Xeui'opteris, Bothrodendron. TJlodendron. Lepidophlois, 

 etcetera. 



The Pottsville and Allegheny time of Xorth America is the approxi- 

 mate equivalent of the Westphalian or Museovian of the Old TTorld and 

 witnessed the maximum development of such genera a^ Alethopteris. 

 Xeuropteris, Sphenopteris. etcetera. The climatic conditions must have 

 been very uniform, for, while there are slight differences, this flora is 

 found to have an almost world-wide range, in fact, as TThite says : 



"The proportion of identical species is so large as to necessitate an extra- 

 ordinary lack of barriers to the freest migration. The flora of the basin of 

 Heraclea in Asia Minor lends itself readily to correlation, stage by stage, 

 with three corresponding formations of the Pottsville in the Appalachian 

 trough; also, of the 33 species reported by Zeillar in a collection from the 

 Westphalian of the Djebel-Bechar region of Russia, 25 are present in the 

 Pottsville of the Appalachian trough." 



With the possible exception of the Jurassic it seems safe to say that at 

 no time in the history of the world has there been such uniform condi- 

 tions and consequent wide distribution of plant life as that of the West- 

 phalian. Thus we iind these practically identical assemblages of plants 

 wide-spread over western Europe, central and eastern Asia, South Africa, 

 eastern Xorth America, and probably southern South America, or froci 

 latitude oS" X. to 25' S. and from longitude 110' E. to 100^° W. 



At the close of Allegheny time and the incoming of Conemaugh or 

 lower Stephanian time there was quite a marked change in the flora that 

 may or may not have been due to climatic change. The gigantic Lepido- 

 dendra had almost disappeared and their places taken by a group of 

 huge tree-ferns, known from the trunks as Psaronius and from the 

 foliage as Pecopteris. This by some is interpreted as evidence of short 

 dry seasons, which seems to be corroborative of the usual explanation 

 of the origin of red beds, that is, as a result of distinct aridity. That 

 there may have been some climatic modification is possible, but the mere 

 presence of red beds can not be accepted as proof positive of aridity-. 

 On this point White ^ writes as f oUows : 



"From the paleobotanical standpoint the widely current belief that aridity 

 in the actual sense is to be assumed as causally and almost Indispensably 



s David White: The origin of coal. Bureau of Mines. Bull. 3S. 1913. pp. 



