156 THE PERMO-CARBONIFEROUS RED BEDS OF 



which could not endure even a sHght change of environment. What the 

 change was it is difficult to say. The Red Bed conditions were apparently 

 continued unchanged into the Triassic. On Poleo Creek, in Rio Arriba 

 County, New Mexico, the author secured a section of a bluff 800 feet high.^ 

 The lower 400 feet or so carried Permo-Carboniferous vertebrates, the upper 

 100 feet Triassic vertebrates; the intervening beds were barren, but with 

 absolutely no indication of a change in the nature of sedimentation or climate. 

 Only at the top of the bluff, above the lowest Triassic fossils, was there a 

 difference in the color and character of the beds. This is typical of the condi- 

 tions over most of North America. In some places, as in western Texas, 

 there is a general difference in the structure, beds, color, and character of the 

 clays and sands, but no great unconformity or evidence of pronounced change. 

 What caused the extinction or displacement of the fauna we can not say — 

 perhaps some slight climatic change, or change in surface, elevation, humid- 

 ity, or food-supply. Any of these would have been sufficient. 



That the North American fauna, or close relatives of some of its members, 

 existed in Europe, in a somewhat modified form, after it had become extinct 

 in the western hemisphere there can be no doubt. This relation between the 

 faunas is, however, a question too large for discussion in this paper. 



» Williston and Case, Jour. GeoL, vol. xx, pp. 1-12, 1912; Carnegie Inst. Wash. Pub. 181, p. i, 1913. 



