I'llYSlCAL CONSIDERATIONS 311 



lii<;liUiii(ls as may have persisted there from the Carboniferous were being 

 eroded throughout the Triassic and Jurassic periods — a time amply suffi- 

 cient for the reduction to a condition of low relief of large land masses. 

 The time during which the Jurassic sea occupied the interior basin was 

 ])robably only a small part of tlie period (14). The character of the 

 Sundance (Jurassic) formation indicates that this sea came in over a 

 well graded area. Later occurred what Emmons has called the Jurassic 

 movement, which expelled this sea from the continent (0, pages 31 and 

 23), and which, in his opinion, should mark the close of the Jurassic 

 period. 



If the Jurassic movement produced any considerable highlands in the 

 Kocky Mountain region, they seem to have been reduced nearly to base- 

 level before Morrison time, for this formation bears evidence, as pre- 

 viously noted (5, page 119), of having been deposited on a nearly flat 

 surface. There were land areas in the Eocky Mountain region somewhat 

 above the level of Morrison deposition, for in some places the formation 

 abuts against their flanks (9, 12, and 13). In a few places conglomerate 

 is found in the Morrison (13 and 8, page 22), but on the whole the 

 lithologic character of this formation indicates the presence in the Rocky 

 Mountain region of lowlands rather than highlands. However, the dis- 

 tribution of the formation indicates that these lowlands were not exten- 

 sive. Cross and Larsen (11, page 238) recognize this condition when, 

 although they found the formation thinning out in some places, they 

 state: "It seems . . . not unlikely that the Morrison beds on the 

 east were connected originally with the Gunnison on the west." 



The physiographic conditions of the time may be pictured as follows : 

 After the long period of degradation that brought the highlands of the 

 Eocky Mountain region to a condition of low relief, this region was up- 

 lifted sufficiently at least to expel the Jurassic sea. After a considerable 

 interval of time, indicated by the extensive overlap of the Morrison, this 

 region seems to have begun a slow general subsidence that resulted in 

 the partial sul)mergence of the interior of North America in the Lower 

 Cretaceous and the more complete submergence in the Upper Cretaceous 

 epoch. During this subsidence the streams deposited their silt farther 

 and farther inland as the gradients were reduced until the lands of the 

 Eocky Mountain region were finally submerged. The resulting fluviatile 

 accumulations — the Morrison — were in turn buried by the sediments 

 (Purgatoire formation) laid down in the encroaching Lower Cretaceous 

 sea, which submerged the Morrison formation in eastern New Mexico and 

 Colorado neai- llio close of Lower Cretaceous time. These Lower Cre- 

 taceous sediments are so closely associated with the Dakota sandstone 

 XXIII — Burx. Geol. Soc. Am., Vol. 20. 1914 



