INTERPRETATION OF ENVIRONMENTAL CONDITIONS 219 



was greatly elevated; indeed, the apparent equivalence of the limestones 

 in the Yukon Territory with those of Alaska seem to indicate that these 

 regions were submerged at the same time, but there is little doubt that the 

 progressive elevation of these lands was from north to south and as little 

 that the Eastern belt was land at least in the Permo-Carboniferous. This 

 elevation was sufficient to furnish much sediment to the west and also to 

 terminate the basins of the Plains and Basin Provinces not far south of the 

 present international boundary. The elevation furnished also the probable 

 route of migration of the Asiatic plants represented by the fern Gigantopteris , 

 which reached as far south as Texas. 



