254 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



West of the Missouri land there was sea during the first part of the 

 late Paleozoic which endured well into the Permo-Carboniferous and only 

 slowly and reluctantly yielded to the prevailing movements of elevation 

 and the accompanying climatic change which culminated in the desiccation 

 of the Triassic. 



Beyond this western sea was the great barrier where the Rocky Moun- 

 tains now stand. There can be no doubt that by the last half of the Penn- 

 sylvanian this barrier was prominent and effective, and one is led to suspect 

 from the amount of material derived from it that it was far larger and broader 

 than has yet been suggested. This barrier was not complete between the 

 Plains and Basin Provinces, for marine and terrestrial deposits of the late 

 Paleozoic may be traced more or less completely around the northern and 

 the southern edges. Though there are short breaks in the connection 

 between the beds on the two sides of the barrier in the south and the char- 

 acter of the sediments changes, it seems very probable that the terrestrial 

 conditions of the eastern side shaded off into marine conditions toward the 

 west, in trans-Pecos Texas. The northern limit of the barrier is equally 

 uncertain. It may have extended north to where the uplift merged into 

 the highlands of the Canadian shield, but the short distance between the 

 sediments of the two provinces in Wyoming and north of the Big Horn 

 Mountains and the probability of continuity in some places indicates that 

 the barrier was broken in several places. The late Pennsylvanian marine 

 deposits of the Basin Province merged to the north into the similar deposits 

 of the great sea of the same age which covered Alaska and western Canada; 

 the Permo-Carboniferous deposits of inclosed and stagnant seas, in the 

 same age, lie upon the southern edge of the land formed by the uplift which 

 drained this sea toward the close of Pennsylvanian time. In greater detail 

 the geography of the continent was approximately as follows : 



In middle Pennsylvanian time the continent of Appalachia extended far 

 to the north and south on the eastern side of North America; its surface, as 

 indicated by the derived sediments, was in general marked by a subdued 

 topography; west of this upland lay a broad area of very slowly sinking land, 

 in which the accumulation of fine sediments kept pace with the subsidence, 

 maintaining a nearly constant but low level of the surface. The rate of 

 subsidence must have been very slow, for the character of the sediments 

 accumulated upon it indicate that the land-surface from which they were 

 derived was so low and gentle that erosion must have been very gradual. 

 For the most part the surface of the sinking area was marked by the presence 

 of great fresh-water swamps, alternating with low elevations and stretches 

 of open water due to local invasions by marine or brackish water. 



The subsiding area west of Appalachia was divided into two distinct 

 parts, approximately the same in position as the Northeastern and the 

 Southern Subprovinces of the Eastern Province, defined previously. The 

 northeastern part, including eastern New England, the Maritime Provinces 



