662 THE AMERICAN NATURALIST. [Vou. XXXII. 
tains elevated in a later Cretaceous period were worn down 
during Tertiary times merely to a gentle topography. 
The other post-Cretaceous changes of this vast region are 
thus summarized by Scott from the results of Pacific coast 
geologists: in the Eocene a long narrow bay occupied the 
great valley of California extending northward into Oregon and 
Washington. At the end of the Eocene or early in the Miocene 
an elevation in California shifted the shore line far to the west. 
In the Miocene the Coast range formed a chain of reefs and 
islands, and at the close an upturning and elevation of the 
mountain range took place, though it became higher afterwards. 
The Coast range sank again early in the Pliocene, and the San 
Francisco peninsula was an area of subsidence and maximum 
deposition forming the thickest mass (58,000 feet) of Pliocene 
in North America. The mountains of British Columbia are 
believed to have been at a higher level than now, as it is sup- 
posed that Vancouver and Queen Charlotte Islands probably 
formed part of the mainland. 
At or near the close of the caus the Sierra Nevada 
increased in height by the tilting of the whole block westward. 
New river valleys, cut through the late basalt sheets of the 
Sierras, are much deeper than the older valleys excavated in 
Cretaceous and Tertiary times, owing to the greater height of 
the mountains and to the consequent greater fall of the streams. 
At this time the Wasatch Mountains and high plateaus of Utah 
and Arizona were again upraised, and the great mountain bar- 
rier of the St. Elias in southeastern Alaska was likewise thrown 
up. At this time also, or perhaps later, the mountains of Brit- 
ish Columbia were probably raised still higher.! It will be seen 
from this that the present topography of the western border 
of our continent, including Central America and the Isthmus of 
Panama, belongs to a new topographic era, and fully substan- 
tiates the view that the fauna of these regions is very recent 
compared with that of the Atlantic border, and that the number 
of nascent or incipient species is much greater. 
4. The Upper Cretaceous Revolution. — Another profound and 
epoch-making change occurred at the beginning of the Upper 
1 Journ. Geol., vol. iv, pp. 882, 894, 897, 898. (Quoted from Drake.) 
