238 The American Geologt8t. October, 1894 
this great erosion is afforded by the range of the Crazy moun- 
tains, which lies immediately north of the Yellowstone river, 
near Livingston, and is conspicuously seen from the Northern 
Pacific railroad. These mountains trend slightly west of 
north, and extend about 40 miles with a width of 15 miles, 
attaining an elevation of 11,178 feet above the sea, and 5,000 
to 6,000 feet above the prairies at their base. Their structure 
has been thoroughly studied by Wolff, who finds that they 
consist of late Cretaceous strata, soft sandstones, nearly hor- 
izontal in stratification, intersected by a network of eruptive 
dikes. The more enduring igneous rocks have preserved this 
range, while an average denudation of not less than one mile 
in vertical amount reduced all the adjoining region to a base- 
level of erosion. The Highwood mountains, about 25 miles 
east of Great Falls, having a bight of 7,600 feet above the sea 
or about 3.500 feet above their base, are described by Davis as 
displaying the same structure and therefore similarl} T testify- 
ing of great denudation. 
The uplift at the beginning of the Tertiary era appears to 
have raised this portion of the plains to a hight above the sea 
as great as the vertical extent of their Tertiaiy erosion, that 
is, to a hight of at least 1.000 to 5,000 feet, increasing from 
east to west. Toward the end of this era the baseleveling 
had reduced the country mostly to a plain which was proba- 
bly only a few hundred feet above the sea, lying much below 
its present altitude. 
Renewed Elevation and partial Baseleveling at the close 
OF THE Tk.KTIARY AND DURING TDK EARLY PART OK TDK QOA- 
TENAttY ERA. 
Between the general Tertiary cycle of baseleveling and the 
Glacial period, there intervened a second great epeirogenic 
uplift, as shown by a return of the conditions of vigorous 
stream erosion and a new cycle of partial baseleveling, by 
which wide flat valleys were cut in the eastern part of these 
Cretaceous plains. In Manitoba the northeastern border of 
the formerly baseleveled expanse was removed, the Creta- 
ceous beds being eroded to the underlying Archean and Pa- 
leozoic rocks upon a large area bounded on the west by the 
escarpment before mentioned as now forming the eastern 
limit of the plains. 
