112 GEOLOGICAL SUEVEY OF THE TEERITOEIES. 



mon for a river to be considerably larger toward its source than at its 

 mouth. Many of the important streams that flow from the Black Hills 

 into the Missouri are lost on their way through the plains. This is 

 especially the case with rivers in the arid regions of New Mexico and 

 Arizona. 



CHAPTER IX. 



OVEE THE FIEST EANGE. 



In the preceding chapter I have endeavored to convey some idea of 

 the remarkable character of the great fresh-water lake basin which 

 occupies so great an area in Nebraska. We have seen that the carbon- 

 iferous limestones pass beneath the lower cretaceous sandstones near 

 the mouth of the Elk Horn, about thirty miles west of Omaha; that the 

 cretaceous rocks extend westward about eighty miles farther, where 

 they aw overlapped by the marls and clays of the White River Group. 

 These form an unbroken mass to the very margins of the first range of 

 mountains west of Cheyenne. Up to this point our ascent has been so 

 gradual that it is hardly perceptible to the common observer, and yet 

 the grade has been upward at the rate of nearly twelve feet per mile. 

 If we examine the excellent profile of the Union Pacific Railroad con- 

 structed under the supervision of the distinguished engineer, General 

 G. M. Dodge, we shall find that Omaha, the eastern terminus of the 

 road, is nine hundred and sixty-six feet above tide-water. At Cheyenne, 

 which is five hundred and seventeen miles west of Omaha, the eleva- 

 tion is six thousand and seventy-two feet ; west of Cheyenne the as- 

 cent increases with great rapidity; at Sherman Station, near the 

 summit of Laramie range, the height is eight thousand two hun- 

 dred and forty- two feet; so that within a distance of thirty-three 

 miles we have a difference of elevation of two thousand one hun- 

 dred and seventy feet, or an ascending grade of nearly sixty-six 

 feet per mile. A profile section across the country east and west 

 from the Missouri River, from the north line to Mexico, would show 

 the same graded ascent, illustrating with great clearness the long- 

 continued but regular upheaval of the great original plateau west of 

 the Mississippi. If we were to stand on the spot where the city of Den- 

 ver is located, five thousand four hundred and thirteen feet above the sea, 

 and look to the north, south, or east, we shall see only a broad, appar- 

 ently level plain, with no perceptible ascent ; but turning our eyes to the 

 westward, the ranges of the main Rocky Mountain chain seem to rise 

 abruptly out of the plain, showing very clearly that when the crust of 

 this great original plateau had been stretched to its utmost tension, these 

 lofty ranges burst through the superincumbent sedimentary strata as the 

 germ breaks through the hard-trodclen earth above it. The series of 

 ridges which are so well exposed along the base of the mountains show 

 plainly that all the formations from the summits of the lignite tertiary to 

 the granites extended uninterruptedly across the area now occupied by 

 these ranges prior to their elevation, and probably up to the close of the 

 cretaceous epoch, and possibly somewhat later. This important fact is 

 better illustrated near Denver than at any point north along the main 

 traveled routes, because the mountains form a portion of the great water- 

 shed of the continent, while the Laramie range west of Cheyenne is a 

 detached portion, seldom rising over eight thousand feet above the sea. 

 This range, however, forms a perfect anticlinal, and must be studied in 



