30 p. B. KING 



many of the basins. In Colorado, such debris in the Animas forma- 

 tion of the San Juan basin and the Dawson arkose of the Denver 

 basin indicates that the nearby San Juan Mountains and Front 

 Range, during the cHmax of their uplift, were heavily overspread by 

 eruptives, nearly all trace of which has now been eroded. 



In the Absaroka Mountains and Yellowstone Park of northwestern 

 Wyoming, violent and explosive volcanism began in late Eocene 

 time, and built up the "early acid breccias" and "early basic brec- 

 cias" of that area (Rouse, 1937, pp. 1262-1272); no doubt their 

 edges once extended eastward over the older Eocene deposits of the 

 Bighorn and Wind River basins. Other flows and breccias were 

 piled over these, probably during later Tertiary epochs, and the hot 

 springs and geysers of Yellowstone Park attest that the volcanic 

 heat has not yet cooled. 



Climax of the eruptions around the Colorado Plateau appears to 

 have been during Miocene time. In the San Juan Mountains of 

 southwestern Colorado where the sequence has been worked out 

 most completely (Larsen and Cross, 1956, pp. 258-260), the Paleo- 

 cene andesitic eruptions were followed by quiescence in Eocene and 

 Oligocene times, when relatively thin, non-volcanic deposits were 

 laid down. On these, during the Miocene, a mile or two of lavas, 

 breccias, and tuffs were piled, but with occasional pauses that per- 

 mitted the cutting of canyons as deep as those today. Lesser erup- 

 tions continued through the Pliocene and into the Pleistocene. The 

 volcanic record of other fields on the periphery of the Colorado 

 Plateau resembles that of the San Juan Mountains, although per- 

 haps less complete and on a smaller scale (Hunt, 1956, pp. 39-53). 



Volcanism in the eastern part of the Cordillera had a significant, 

 though secondary role in the shaping of the geography. Eruptions 

 in the larger fields much increased the relief, although upbuilding 

 was somewhat compensated by subsidence under the load of erup- 

 tives and by transfer of magmas from their subsurface reservoirs to 

 the surface. Volcanism changed the regimen of streams by loading 

 them with detritus, and in places dammed and diverted their courses. 

 In some places these effects may be read plainly; in others, where the 

 eruptives have largely been removed by erosion, the effects are 

 difficult to assess. 



Contrast between Eocene and Present Conditions. Compare the 



