MESOZOIC AND CENOZOIC GEOGRAPHY 469 
were formed, the mountain elevation was accompanied 
by an uplift of plateau and neighboring plain. This 
mountain formation involved the entire western part 
of the Mississippi valley, and raised above the sea the 
old ocean bottom of Cretaceous times. 
Accompanying the growth of these western moun- 
tains, there was a notable development of volcanoes 
throughout the west, from Canada to Mexico. These 
continued to erupt throughout the next period; and 
as a result of their action, the Cordilleras of the 
west, and their included plateaus, are the site of 
remains of volcanic cones and of extensive lava 
flows (see pp. 330 and 350). 
The mountains rising above the sea, at first trans- 
formed the west into a mountainous mass of land, 
with intermediate arms of the sea, somewhat like 
the present condition of the eastern coast of Asia. 
Further development of the mountains cut off these 
arms, and gradually transformed them to inland seas, 
at first like the Mediterranean, Black, and Caspian 
seas, and then like our Great Lakes. 
It is interesting to note that the great Mississippi valley, which 
now forms such an important feature in the geography of the 
country, had its position practically determined in the very 
earliest geological ages. Bounded on three sides by mountain 
masses, it was at first all sea; then as these mountains devel- 
oped, and the sea bottom was transformed to dry land, the 
central part of the basin still remained lowland, into which 
