544: CENOZOIC ERA— AGE OF MAMMALS. 



Eocene lakes were subject to prodigious erosion, and much of the gen- 

 eral erosion of the Plateau region occurred at that time. At the end of 

 the Miocene occurred the greatest event of the Tertiary period, one of 

 the greatest in the history of the American Continent. At that time 

 the sea-bottom off the then Pacific coast was crushed together into the 

 most complicated folds (p. 259), and swollen up into the Coast Cham, 

 and at the same time fissures were formed in the Cascade Eange, with 

 the outpouring of the great lava-flood of the Northwest, already spoken 

 of (pp. 210, 262). Coincidently with this there was a further letting 

 doivn of the region of the Plains and of the Basin, and a consequent 

 extension of the Pliocene lakes in these regions, attended probably with 

 a further rise of the Plateau region. During the Pliocene, the greater 

 part of the canon-cutting of the Plateau region, and nearly all the great 

 lava-flows of the West, took place. At the end of the Tertiary, these 

 lakes were in their turn obliterated by the further upheaval of the con- 

 tinent, which inaugurated the Quaternary. Coincident with this gen- 

 eral uplift, mountain-making by crust-block tilting occurred on a grand 

 scale. The Sierra, the Wahsatch, and the Basin Ranges assumed their 

 present form and height (p. 265), and the great north and south 

 fault-cliffs of the Plateau region were mainly formed. 



While this was going on in the 'western portion of the continent, on 

 the southeastern and southern border the continent gained, by gradual 

 rise, nearly all the area shaded as Tertiary. In this direction the con- 

 tinent was finished with the exception of a large portion of Florida 

 and the sea-islands and alluvial flats * about the shores of the Southern 

 Atlantic and Gulf States. These belong to a still later period. 



Thus we see that from the end of the Cretaceous to the end of the 

 Tertiary there was a gradual upheaval of the whole western half of the 

 continent, by which the axis, or lowest line, of the great interior con- 

 tinental basin was transferred more and more eastward to its present 

 position, the Mississippi River. Probably correlative with this up- 

 heaval of the western half of the continent was the down-sinking of 

 the mid-Pacific bottom, indicated by coral-reefs (p. 155). Also as a 

 consequence of the same upheaval the erosive power of the rivers was 

 greatly increased, and thus were formed those deep canons in the 

 regions (Xew Mexico, Colorado, and Arizona) where the elevation was 

 greatest. Thus the down-sinking of the mid-Pacific bottom, the dodily 

 ajjheaval of the Pacific side of the continent, and the doivn-cutting of 

 the river-channels into those wonderful canons, are closely connected 

 with each other. 



* In some places about the shores of the Gulf, for reasons which will be explained 

 hereafter, the Quaternary deposits are considerably elevated above the sea-level. 



