154 DAVID WHITE 



with the return of some of the northern types to portions of Gondwana 

 land, and, secondly, with the fitness of some of the Gondwana types, 

 especially Glossopteris, Phyllotheca, and Noeggerathiopsis, not only 

 to join with Walchia and Callipteris in the Zechstein flora of northern 

 Russia, but later to meet with Podozamites, Ctenophyllum, Cladophle- 

 bis, Clathropteris, and Dictyophyllum, under the mild climatic con- 

 ditions of the early Mesozoic. Glossopteris, perhaps most adaptable 

 of the older Gondwana types, was able to reach as far as the supposed 

 Rhetic of Tonquin where it is last seen. 



Glaciation. — It appears, therefore, that the interval, or better the 

 intervals, of glaciation in Gondwana land were of relatively short 

 duration as compared to all Permian time, though the thickness and 

 distribution of the glacial deposits indicate a magnitude far exceeding 

 that of Pleistocene glaciation. At the same time it seems improbable 

 that refrigeration could have occurred in so many regions of the 

 southern and eastern hemispheres, even approaching the equator, 

 without some corresponding, though unequal, changes of world-wide 

 extent. Concerning this very important point, the evidence is quite 

 inconclusive and the opinions varied. 



American Permian types derived from Europe. — If we compare the 

 plants of eastern America with those of western Europe we find the 

 greater changes in Europe where the extensive orogenic movements 

 and attendant shifting of basins of deposition doubtless stimulated 

 the evolution of the Permian flora. On the other hand, in the Appa- 

 lachian trough, where environment was but little affected by orogeny, 

 and where sedimentation was uninterrupted, there is only gradual 

 change, many of the Stephanian types persisting far up in the Dunkard 

 formation. The relatively few species characteristic of the Euro- 

 pean Permian which occur in the Dunkard, and which were not able 

 to conquer the older flora under the conditions then existing, are 

 clearly migrants from western Europe. It must be noted, though, 

 that both West Virginia and Kansas exhibit new generic types, the 

 products of local conditions, that have not been found outside of these 

 regions. On the other hand, Walchia, which is present in Kansas 

 and New Mexico has not yet been discovered in the Appalachian 

 trough though it is present in the Nova Scotian basin, which seems 

 to have been in closer touch with Europe at this time. The floral 



