STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 69 



self and others in New Alexico and Colorado. He recognizes pene- 

 planation in the southern Rocky-Mountain region prior to the begin- 

 ning of the Upper Cretaceous. The evidence all indicates that the 

 interior continental sea extended from Utah and Arizona eastward 

 over the present site of the Rocky Mountains. 



The source of sediments was at the south and west, as appears 

 from discussions by Lee, Stone and Calvert and Stebinger,-* as 

 well as from sections by many other observers. The coarser ma- 

 terials are in the southern and western parts of the area, while, 

 toward the east, land and border-land conditions disappear, so that 

 the rocks become shales with more or less of limestone. But toward 

 the close of the Cretaceous, land and shore deposits extended far 

 east, indicating perhaps a long period of comparative stability prior 

 to the great mountain-making period of the Tertiary. The vast 

 area, reaching in some places almost to the ^Mississippi, was ap- 

 parently at first almost a peneplain, over which the early Cretaceous 

 sea advanced to the western border. 



During and after the Rocky-Mountain revolution, erosion was 

 so energetic that, in New Mexico, Arizona, Utah and Colorado, the 

 Cretaceous was broken into isolated "fields" or "basins," separated 

 in many cases by ranges showing i\rchean rocks at thousands of 

 feet above the general altitude of the region. But this greatly dis- 

 turbed area becomes narrower toward the north, so that, in much of 

 Wyoming, the continuity is broken only by comparatively short 

 ridges around which the Cretaceous rocks outcrop. Still farther 

 north, the undulations in by far the greater part of the area are 

 gentle and sedimentation appears to have been continuous into the 

 Tertiary ; the greatly disturbed region on the western side trends 

 toward the northwest and becomes very narrow. During the Cre- 

 taceous, deposition was practically continuous, there being only local 

 unconformities, so small vertically and horizontally as to be sur- 

 prising, in view of the vast area under consideration. There are, 

 however, great variations in thickness which seem to be due to dififer- 

 ential subsidence. The conditions favoring accumulation of coal 

 were repeated many times in the region of coarser sediments and 



24 W. T. Lee, Prof. Paper. 95-C ; R. W. Stone and W. R. Calvert, Econ. 

 Geol, Vol. v., 1910; E. Stebinger, Prof. Paper, go-G, 1914. 



PROC. AMER. PHIL. SOC. , VOL. LVI, F, MAY 23, I917. 



