458 THE JURASSIC PERIOD 



ranges of the Appalachian Mountains were much wasted away, 

 and the newly upheaved, tilted, and faulted beds of the Trias were 

 deeply eroded. 



In the interior region of the continent the course of events was 

 different. The Colorado island became joined to the mainland, 

 forming a peninsula. In the great area from the Uinta Moun- 

 tains southward to New Mexico and Arizona, and extending from 

 the Colorado peninsula westward to the Great Basin land, was an 

 interior sea or salt lake, in which were laid down great thicknesses 

 of barren sandstone, without fossils. These beds are placed in 

 the Jurassic entirely upon stratigraphic grounds, and whether they 

 represent the whole Jura, or only a part, and if so, what part, are 

 questions that cannot at present be answered. West of the Great 

 Basin land, the Lias is found in California and Oregon, but not as 

 yet in British Columbia, and recurs in several of the Arctic islands. 

 The faunal relations of this Pacific border Lias are with that of 

 central Europe. 



The Middle Jurassic of North America is still very little known, 

 but appears to have much the same distribution as the Lias. 



In the upper Jura some extensive geographical changes occurred. 

 Early in this epoch a depression was formed in the northern inte- 

 rior region, allowing the waters of the ocean to flow in and estab- 

 lish a gulf over portions of what had been the Triassic inland sea. 

 The boundaries of this great gulf are not yet fully known, but it 

 extended over the northern portion of Utah, where the Wasatch 

 and Uinta Mountains now are, and covered most of Wyoming, 

 as far east as the Black Hills, together with southern Montana. 

 Although Jurassic exposures have not been found in Canada east 

 of the Cordillera, yet the fossils of this gulf show that its waters 

 were completely separated from those of California, which still re- 

 tain their European connection, and were probably derived from a 

 transgression from the north or northwest. The sediments depos- 

 ited in the gulf are largely limestones, though with much clay shale 

 and marl, while the presence of gypsum points to the existence 

 of lagoons. The beds attain their maximum thickness of about 

 1800 feet in the Wasatch Mountains, which then formed the east- 



