38 The American Geologist. •^"^^'' ■^^'^'• 
Around the Crazy mountains, prominently seen from Liv- 
ingston, where the Northern Pacific railway branch for the 
Yellowstone Park leaves the main line, much deeper general 
erosion of the Plains has taken place, to the vertical extent of 
3,000 to 5,000 feet. This group of mountains, about 30 miles 
long from south to north and 10 to 20 miles wide, rises in its 
highest peak 11,178 feet abo\-e the sea, being 5,000 to 6,000 
feet above the adjoining prairies. The structure of this moun- 
tain mass has been thoroughly studied by Dr. J. E. Wolff, who 
finds that it consists of late Cretaceous or early Eocene strata, 
mostly soft sandstones, nearly horizontal m stratification, named 
the Livingston formation, intersected by central volcanic out- 
flows and a network of innumerable radiating dikes." The 
more enduring igneous rocks have preserved the mountain 
group, while an average denudation of nearly or quite one 
mile in vertical amount reduced all the surrounding country 
to a baselevel of erosion. Alluvial sedimentation on this area 
was rapid and deep while the neighboring Rocky mountain 
ranges, west of these Plains, were being uplifted ; and the en- 
suing Tertiary erosion in baseleveling here greatly exceeded its 
volume from the country eastward. 
A hundred miles distant thence to the north, the Highwood 
mountains, about 30 miles east of Great Falls, having a bight 
of 7,600 feet above the sea or about 3,500 to 4,000 feet above 
their base, are described by Prof. W. M. Davis as displaying a 
similar structure, and therefore testifying likewise of great 
denudation, t 
It seems a reasonable estimate that the average depth of 
erosion from all this northern part of the Plains, stretching 
from the Red river valley to the Rocky mountains, is at least 
1,000 feet. Such a vast volume of detritus was carried away 
by the predecessors of the Missouri and Saskatchewan rivers 
and their tributaries during the Tertiary era, to be mostly 
borne forward to the sea by the great streams which represent- 
ed the Mississippi and Nelson rivers during that time. Thus 
it is seen that, if the Tertiary era had a duration of about 3,- 
000,000 or 4,000,000 years, as estimates of the ratios of geolo- 
• Bulletin, Geol. Society of America, vol. iii, 1S92, pp. 4-4-5-452. 
t Mining Industries of the United States. Tenth Census, vol. xv, pp. 710, 
737, 745. See also the U. S. Geologic Alias, Folios 1, 56 and 55, respectively 
the Livingston, Little Beit Mountains, and Ft. Benton Folios, by Walter H, 
Weed, mapping and describing these two mountain groups. 
