CHANGES AT CLOSE OF JURASSIC PERIOD. 167 



which compose the Jura in the northern Rocky Mountains are very similar 

 to their representatives in the Hills, differing chiefly by carrying more car- 

 bonate of lime, while at the south a considerable portion of the formation 

 assumes the habit of the Red Beds, abounding in vivid colors and carrying 

 large quantities of gypsum. 



The close of the Jurassic age witnessed great continental and oro- 

 graphic changes in what is now the western part of the continent. The 

 Sierra Nevada was uplifted and so were the ranges of the great interior 

 basin, and the continent was so elevated that the succeeding Cretaceous 

 made no deposit between the east side of the Wasatch and the west side of 

 the Sierra Nevada. In the Mississippi Valley, too, there must have been 

 elevation and erosion either then or before, for Trias and Jura are absent and 

 the Cretaceous rests on a worn surface of Carboniferous. But in the region 

 of the Black Hills there was no emergence. The Dakota sandstone rests 

 in perfect conformity on the Jura, and although the change in the character 

 of the sediments is great, it is not abrupt. Probably in the magnitude of 

 the change from soft calcareo-arenaceous clays below to coarse-grained 

 sandstone above we have the only local evidence of the revolutions of the 

 time. 



The physical conditions which prevailed during the deposition of the 

 Jura were a continuation of the shallow water of the Triassic age, with 

 probably a somewhat deeper submergence. The marine type of the fossil 

 forms indicates that there was a free communication with the ocean, and 

 hence that the period was one of a shallow marine submergence over the 

 Rock}- Mountain area. 



The fossil fauna of the European Mesozoic (Trias, Jura, and Creta- 

 ceous) includes so many species and individuals of huge aquatic reptiles 

 and the reptilian type characterizes so largely the vertebrate remains, that 

 the age is known as the Reptilian age. The saurians seem to have reached 

 their maximum development during the Jura, though they are also found 

 abundantly in the older Trias and the later Cretaceous. In the American 

 Trias of the East have been found those great foot-tracks of the Connecti- 

 cut Valley, now considered to have been made by huge batrachians; but 

 in the Jura of the Far West no vertebrate remains have yet been discovered, 



