THE MESOZOIC GEOGRAPHY. 277 



the interior region of our continent shows that there were two principal 



meridional lines of depression in the earth's surface at that time, the one in 

 the region of the Great Plains to the east of the Rocky Mountain front and 

 the other to the east of the Wasatch uplift, each of which probably extended 

 north beyond the Canadian boundary. The western continent beyond the 

 Wasatch mountains had its greatest east and west extension between the 

 40th and 45th parallels of north latitude, the Mesozoic ocean extending 

 further westward both to the north and south of this continent and possibly 

 connecting beyond our boundaries with that on the Pacific slope. It is 

 probable, therefore, that in these middle latitudes the general level of the 

 country, as represented by its plains and valleys, was higher than in the 

 more northern and southern regions, the bottoms of the principal depressions 

 having a general slope northward and southward toward the present oceans. 



The general elevation that accompanied the Jurassic movement therefore 

 raised the whole interior region above the ocean, while the dynamic move- 

 ments produced the effects already noticed within the Rocky Mountain 

 region, and also raised a barrier which kept out the waters of the southern 

 ocean, or Gulf of Mexico, from the eastern and partially, or possibly entirely, 

 from the western meridional depression. 



During the elevation a fresh-water lake, whose extent is as yet imperfectly 

 defined, accumulated behind this barrier. It filled the valleys of the Rocky 

 Mountain region and extended north as far as the Black Hills. It must 

 have filled a portion at least of the Great Plains depression, but its western 

 shore-line is now buried beneath Cretaceous deposits and may never be accu- 

 rately defined. The extent of fresh-water Jurassic beds on the south and 

 west of the Rocky Mountain region will, however, probably be determined in 

 future examination of the region. At present it can only be said that fossils 

 apparently belonging to this horizon are said to have been found in north- 

 ern New Mexico by Newberry on the south, aud on the banks of the Green 

 river in Wyoming by Steward, of Powell's party, on the west. 



During the gradual subsidence which followed this elevation the barrier 

 was being eroded, and an outlet may have been formed through which the 

 Jurassic lake was drained, so that no further deposition went on in its bed 

 until it was again invaded by the ocean ; though, as far as present evidence 

 goes, the subsidence was not sufficient to admit the w r aters of the ocean 

 within the Rocky Mountain region until Dakota times. Marine water-, 

 however, must have entered the western depression from the north in Brit- 

 ish Columbia to admit the deposition of the Kootanie series of beds, and it 

 seems not improbable that marine Cretaceous beds below the Dakota may 

 yet be found in the western depression to the south, in the Plateau province. 



That a certain amount of erosion of the fresh-water Jurassic beds after 

 the drainage of the lake may have taken place in the Rocky Mountain 



