AREAL GEOGRAPHY OF NORTH AMERICA IN THE LATE PALEOZOIC 259 



ments has prevented the occurrence of fossils or some obscure factor pre- 

 vented the occurrence of animals in this large area. 



In this connection it is pertinent to recall that the size and nature of 

 the barrier between the Plains and Basin Provinces is still little understood. 

 The widespread marine sediments of late Pennsylvanian time and their 

 great thickness strongly suggest a suppression of the Rocky Mountain axis 

 at that time and it is the opinion of some writers that the whole region was 

 entirely submerged. On the other hand, the amount of Permo-Carbon- 

 iferous sediment is so great that it can only have come from a very large 

 area and the nature of the material implies considerable height and vigorous 

 erosion. The change in the sedimentary record reveals an important uplift 

 forming an elevation of a height and geographical extent for which we have 

 no other evidence. 



There is some good reason to believe that the barrier between the two 

 provinces failed in the region of the Black Hills and Bighorn Mountains. 

 The Permo-Carboniferous red beds of the Black Hills unquestionably belong 

 to the Plains Province and the equivalent red beds of the western side of the 

 Bighorn may be traced with little question into the phosphate-bearing 

 beds of the Basin Province. If the barrier continued to the north it must 

 have passed between the two elevations, but it is very probable that the 

 elevations were far less prominent in the late Paleozoic than now and that 

 they were nearly, if not quite, covered by the sediments of the time. If the 

 red sediments are continuous between the two, the provinces were united 

 at the northern end and the source of much of the material was from the 

 permanent land to the north and east. 



At their southern ends the two provinces are separated by the great 

 mass of late Pennsylvanian limestone, now thrown up into the series of low 

 mountain ranges which run through central New Mexico. Whatever may 

 have been the condition in Permo-Carboniferous time, there was clearly an 

 effective barrier to the migration of vertebrate land animals, for the reptiles 

 and amphibians found near Soccoro and Abiquiu are distinctly different 

 from those found in the Plains Province. There can be no doubt that in late 

 Paleozoic time the western part of trans-Pecos Texas and the adjacent 

 portions of Mexico and New Mexico were covered by a sea which extended 

 into Arizona. This sea left the great Guadalupian series of limestones, 

 the upper part of which is Permo-Carboniferous, possibly true Permian, 

 and the sea may have extended to the north, constituting a temporary but 

 effective barrier to land life. It is true that the upper limestone of the 

 Guadalupian series shades off into red deposits toward the north, implying 

 land in that direction. In the absence of any definite evidence of a high 

 mountain barrier, the presence of a temporary arm of the sea would be the 

 most satisfactory explanation of the separation of the two faunas ; it would 

 constitute an efficient continuation of the land barrier farther north. 



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