68 GEOLOGICAL SURVEY OF THE TERRITORIES. 



ain-ridges, the ascent coutimies to increase, and when we reach the val- 

 ley of Dearborn River, 448 miles farther west, we ascertain that this 

 locality is 4,091 feet above the sea-level, and that in the distance of 448 

 miles we have ascended 2,081 feet, or nearly five feet to the mile. The 

 valley of Dearborn River is just at the foot of the mountains, and to that 

 point the country traversed belongs to the true type of the western 

 prairie. Again, if we examine the profile, commencing at Council Bluffs, 

 on the Missouri Eiver, we find the elevation at that point to be 1,327 feet 

 above the sea-level. Thence proceeding westward to the sources of 

 Lodge Pole Creek, at the base of the Laramie range of mountains, we 

 have made an ascent, while thus passing over the prairie region, of 

 nearly 5,000 feet. We thus see that, in the distance of 550 miles, we 

 have reached an elevation of 3,000 feet higher than our starting-point, 

 by an ascent of five feet to the mile. 



Again glancing at the profile extending from Fort Leavenworth west- 

 ward, we observe that at the Missouri Eiver the elevation is 904 feet 

 above the sea. At the base of the Laramie range of mountains, 059 

 miles west, the elevation is 6,716 feet. To illustrate the increased ra- 

 pidity of ascent as we approach the vicinity of the upheaved ridges, we 

 see that the elevation at the forks of the Platte is 3,000 feet above the 

 sea, making an ascent from the Missouri River to this point, a distance 

 of 413 miles, of 2,096 feet, or about five feet to the mile. From the forks 

 of the Platte to the foot of the Laramie Mountains, a distance of 246 

 miles, we find au increased elevation of 3,716 feet, or 15 feet to the mile. 

 After reaching the base of the elevated ridges, the ascent is more or less 

 abrupt, sometimes risiug to the height of 3,000 to 6,000 feet above the 

 open prairie country around. 



We might continue our remarks, in regard to the profiles, still farther 

 southward, with similar results, but we have said, enough to indicate the 

 beautiful unity in the physical development of the western portion of 

 our continent. We have shown that the whole country west of the 

 Mississippi to the Pacific maybe regarded as avast plateau, and that it 

 was gradually elevated until the crust of the more central portions was 

 strained to its utmost tension, and that it then burst, and slowly were 

 evolved the lofty ranges which, taken collectively, soon pass under the 

 name of the Rocky Mountains. 



So far as my own observations have extended, there appear to be two 

 types of mountain-elevations, namely, those elevations which have a 

 granite nucleus and form long continuous lines of fracture with far less 

 inequality of outline, and those ranges which are composed of erupted 

 rocks, which are very rugged in their outline and irregular in their trend. 

 The Black Hills, the most eastern outlier of the main mountain-range, 

 present an excellent illustration of the first type. Very little was known 

 of these mountains until they were explored in the summer of 1857 by 

 au expedition placed, by the War Department, under the command of 

 Lieutenant G. K. Warren, United States Army, to which expedition the 

 writer was attached as geologist and naturalist. A preliminary report 

 of the results of this exploration was presented to the War Department 

 under the title '' Explorations in Nebraska and Dakota in the years 

 1855, 1856, and 1857." 



The Black Hills lie between the 43d and 45th degrees of latitude and 

 the 103d and 105th parallels of longitude, and occupy an area about 

 100 miles in length and 60 in breadth. According to Lieutenant War- 

 ren, " the shape of the mass is elliptical and the major axis trends about 

 20° west of north. The base of these hills is 2,500 to 3,000 feet above 

 the sea, and the highest peaks 6,700 feet." The whole range is clasped^ 



