504 MESOZOIC TIME — REPTILIAN AGE. 



great, raising out of the sea a large part of the Kocky Mountain 

 region. About Santa Fe in New Mexico, the Cretaceous beds are 

 6000 and 7000 feet above the sea ; near Pike's Peak, 4500 feet ; at 

 Deer Camp on the North Platte, nearly 0000 feet; on the Big Horn 

 Mountains, 6000 to 7000 feet ; about Fort Benton on the Upper 

 Missouri, and westward along the base of the mountains, 4000 to 

 6000 feet ; in the vicinity of the Wind Eiver Chain, 6800 feet. 

 Hence, the whole rise since the close of the Cretaceous about the 

 central region of the Kocky Mountains has amounted to nearly 

 7000 feet ; and from this it decreased eastward towards the Missis- 

 sippi and westward towards the Pacific. What part of this eleva- 

 tion in these mountain-regions took place immediately after the 

 Mesozoic era, and how much later, is not easily ascertained. Part 

 was unquestionably of later date, as Hayden has shown. This sub- 

 ject comes up again under the general observations on the Cenozoic. 



The elevation of the great plateau appears to have been a gra- 

 dual upward movement without much disturbance of the rocks. 

 Through 3000 or 4000 feet, nearly to the base of the summit-moun- 

 tains, as Hayden observes, the beds are nearly or quite horizontal. 

 But along the sides of the mountains there is some dip, though 

 usually small. On both the east and west sides of the Wind Eiver 

 Chain, and about the Big Horn Mountains, the dip does not exceed 

 one or' two degrees. West of the Black Hills, however, and in 

 some other localities, the beds incline 10° to 25°. 



Causes of the Destruction of Life. — The complete extermination of 

 species at the close of the Cretaceous period has not been fully ex- 

 plained. It was probably connected with the great changes of 

 level which took place at the time, as has been shown, over the 

 Eastern and Western continents. The elevations to the north may 

 have been even greater than has been supposed ; for elevations 

 do not leave as indubitable a record as subsidences. In North 

 America there are no Tertiary beds known north of southern New 

 England on the east, and none in the Arctic, — indicating, appa- 

 rently, that the whole area was above the sea then, as now. The 

 emergence of the continents would have extinguished the life of 

 the continental seas ; and a large increase of land in the higher 

 temperate and polar regions would have given completeness to the 

 destruction by causing a colder temperature in both the air and 

 the waters. It is therefore most probable that the destruction was 

 due (1) to the more or less complete emergence of the continents 

 and accompanying elevation of mountain-ranges; and (2) to a 

 change of climate and oceanic temperature, — both the air and 

 oceans being rendered colder than in the Mesozoic era. 



