36 The American Geologist. •^"'^'' ^^'^"'■ 
west by denudation ; but the surface yet retains so much sem- 
blance of its original flatness as to be commonly called "The 
Plains." Like all the interior basin drained by the Mississippi, 
Missouri, St. Lawrence, and Xelson rivers, between the Ap- 
palachian and Cordilleran mountain belts, the Plains, 800 miles 
wide and of much greater extent from south to north, have 
been exempted from the throes of mountain building. Their 
only oscillations of altitude have been epeirogenic, in marked 
contrast with the "grand orogenic movements which formed the 
Cordilleran ranges. 
At the beginning of the history of the Plains, one of the 
mighty mountain-building and continent-making epochs gave 
rise to the principal ranges of the Rocky mountains, the frontal 
parts of the Cordilleran belt, which were folded and uplifted 
near the end of Cretaceous time. As the chief orogenic revolu- 
tion producing the Appalachian belt of mountains, from north- 
ern Alabama to New Hampshire and Maine, coincided with 
the close of the Paleozoic era, so the end of the ]\Iesozoic era 
witnessed the upheaval of the sea bed to form the Great Plains, 
the birth of the Mississippi flowing at the foot of their east- 
ward slope, and the thrusting up of mountain ramparts along 
all their western border. The sites of Helena, Butte, and Great 
Falls, cities of the mountains and plains of Montana, then 
emerged from 
"The stillness of the central sea." 
Ten years ago I published a paper from my studies of "Ter- 
tiary and Early Quaternary Baseleveling in Minnesota, ]\Iani- 
toba, and Northwestward,"* and ever since I wished to cross 
the western half of our continent, until opportunity came last 
year. In my journey over the Plains and the broad Cordilleran 
region of mountains, valleys, and basins, the vastness of Ter- 
tiary erosion was more fully appreciated, and I was impressed 
with the multitude of the mountain ranges, rather than by their 
liight. 
In Montana, Idaho, and Washington, these mountains are 
of the same order, in respect to altitude above the land at their 
base, as the White, Green, and Adirondack mountains, in- 
stead of representing the most lofty mountains of all the world, 
* Amer. Gbologist, vol. xtv, pp. 235-246, Oct , 1894; BuUetia, Geol. Soc. 
of America, vol. vi, 1894, pp. 17-20. 
