116 ON THE GEOLOGY AND NATURAL HISTORY 
range of mountains, 659 miles west, the elevation is 6716 feet. To show the increased 
rapidity of ascent as we approach the vicinity of the upheaved ridges, we see that the 
elevation at the forks of the Platte is 3000 feet above the sea, making an ascent from the 
Missouri river to this point, a distance of 413 miles, of 2096 feet, or about 5 feet to the 
mile. From the forks of the Platte to the foot of the Laramie mountains, a distance of 
413 miles, we find an increased elevation of 3716 feet, or 15 feet to the mile. After 
reaching the base of the elevated ridges, the ascent is more or less abrupt, sometimes 
rising to the height of 3000 to 6000 feet above the open prairie country around. We 
might give many more illustrations similar to those just stated, extending them over to 
the Pacific coast, but we reserve them for a future occasion. We have said enough, how- 
ever, to indicate the beautiful unity in the physical development of the western portion of 
our continent. With the above brief outline of a subject which I hope to pursue much 
farther at some future time, I proceed to consider the geological structure of the moun- 
tain chains. 
We may at once make the statement, that there appears to be two types of mountain 
elevations, namely, those elevations which have a granite nucleus and form long continu- 
ous lines of fracture with far less irregularity of outline, and those ranges which are com- 
posed of erupted rocks, which are very rugged in their outline and irregular in their trend. 
We will in the first place examine the Black hills as an illustration of the first type, which 
is the most eastern outline of the main mountain range. Very little was known of these 
mountains until they were explored in the summer of 1857, by an expedition placed by 
the War Department under the command of Lieut. G. K. Warren, T. E., U.S. A., to 
which expedition the writer was attached as geologist and naturalist. A preliminary re- 
port of the results of this exploration was presented to the War Department under the 
title of “‘ Explorations in Nebraska and Dakota, in the years 1855, 6, and 7.” 
The Black hills lie between the 43d and 45th degrees of latitude, and the 103d and 
104th degrees of longitude, and occupy an area about 80 miles in length, and from 30 to 
50 miles in width. According to Lieut. Warren, the shape of the mass is elliptical, and 
the major axis trends about 20° west of north. The base of these hills is about 2500 to 
3000 feet above the sea, and the highest peaks 6700 feet above the ocean. The whole 
range is clasped, as it were, by the north and south branches of the Big Shyenne river, the 
most important stream in this region. ‘The north branch passes along the northern side 
of the range, receiving very many of its tributaries and most of its waters from it, but 
takes its rise far to the westward of the range, near the sources of Powder river, in the 
“divide” between the waters of the Yellowstone and those of the Missouri. 
The south fork also rises in the same divide, flowing along the southern base of the 
range, and also receives numerous tributaries which have their sources in it. These two 
