360 HISTORICAL GEOLOGY. 



Disturbances which closed the Cretaceous Period 

 and Mesozoic Era. Remember that during the Creta- 

 ceous a great sea, stretching from the Gulf of Mexico to 

 the Arctic Ocean, covered the whole Plains and Plateau 

 region, and divided the continent into two continents 

 an eastern and a western. Now, at the end of the Creta- 

 ceous, this great sea was abolished by the gradual upheaval 

 of this region, and the continent became one. At the 

 same time the western marginal bottom of the great 

 interior sea yielded to horizontal pressure, and was 

 crushed together and swelled up into the Wahsatcli 

 Range. At the same time, also, the Colorado Moun- 

 tains, which had been a line of islands in the Cretaceous 

 sea (map, page 349), were pushed up, and the Cretaceous 

 strata sharply uptilted on the flanks. At the same time, 

 also, the Uintali Mountains seem to have been born. 

 Such great changes in physical geography imply corres- 

 ponding changes in climate, and in fauna and flora. We 

 ought to, and do, indeed, find the animals and plants 

 very different in the next age (Cenozoic). 



Laramie or Transition Epoch. 



The abolition of the great Cretaceous sea, and the 

 unification of the continent, as we have said, were pro- 

 duced by the upheaval of the Plains and Plateau region. 

 When completed, the Plateau region was occupied by 

 great fresh-water lakes, which we shall describe hereafter. 

 But this change took place gradually, passing through 

 intermediate stages of brackish-water seas. When marine 

 conditions prevailed, it was undoubtedly Cretaceous ;, 

 when fresh-water conditions were established, it was 

 undoubtedly Tertiary. But what shall we cull the inter- 

 mediate time of brackish water ? This is evidently a 

 transition period. It is the lost interval between the 

 Cretaceous and the Tertiary in Europe, recovered here. 



