GIRTY 



sediments, and by the occurrence in them of pebbles containing 

 Pennsylvanian fossils. On this account it is tentatively assumed 

 that the line of division between the Eastern and Western 

 provinces passes through western Texas, central or eastern 

 New Mexico, western Colorado, and so on upward, in a north- 

 western direction, following nearly the trend of the Rocky 

 Mountains. This matter, however, like that of the correlation 

 and dispersion of the Pacific faunas, is left open to revision as 

 new facts are added and as the mass of evidence now at hand 

 is subjected to critical comparison and analysis. 



The continental sea in Mississippian time, however, proba- 

 bly spread as far west as Nevada, and had almost the same 

 limits during the Pottsville epoch, neither group of faunas, so 

 far as known, having penetrated to the Pacific coast. But it 

 would appear that during Pennsylvanian, and also probably 

 during Permian time, its western term was fixed much farther 

 east, its contracted limits favoring shallower depths and marginal 

 conditions upon its eastern shore suitable for the formation of 

 coal, and correspondingly unsuitable to marine life. To the 

 west, beyond the hypothetical barrier, material or intangible, 

 the unimpeded waters probably spread afar, and the faunas 

 which they supported have much in common with those of 

 Asia and eastern Europe. 



The differences presented b}' these western faunas, when care- 

 fully compared with those of the Mississippi valley, are real and 

 important, and the explanation suggested above is that during 

 the later portions of Pennsylvanian time, they were developed 

 in different provinces. If this explanation be rejected it appar- 

 ently follows that the differences are due to geologic horizon 

 rather than to geographic position. It seems almost impossible 

 that the two series of faunas can be equivalent without belong- 

 ing to different provinces, and very improbable that the eastern 

 one overlies the western. On the hypothesis that they are co- 

 provincial, therefore, to the column of Pennsylvanian rocks 

 found in the Mississippi valley must be added a great series 

 whose development was western and the facies of whose fauna 

 is Asiatic. This series is not found east of the Rocky moun- 

 tains unless it proves to be represented by the more or less un- 



