DESCEIPTIVE GEOLOGY. 



SECTION I. 



COLOEADO EANGB. 



BY ARNOLD HAGUE. 



Physical Aspect of the Geeat Plains. — To the eastward of tlie 

 first range of the Rocky Mountains, the Great Plains stretch for several 

 hundred miles, with gentle easy slopes, toward the Mississippi Valley. By 

 reference to Map I, accompanying the report of the Geological Exploration 

 of the Fortieth Parallel, it will be seen that the eastern boundary of the 

 survey lies along the meridian of 104° 30' west from Greenwich, extending 

 out upon the Great Plains only 30 to 65 miles beyond the foot-hills of 

 the Colorado Range, so that only a comparatively narrow strip comes 

 within the limits of our observation. To the eye, they present either a level, 

 plateau-like country, or a broad, gently undulating surface, relieved only by 

 low, smooth ridges, with abrupt bluffs along main drainage-channels, usually 

 in an east and west course; or the ends of long horizontal table-lands, with 

 escarpments from one to two hundred feet above the plain below. 



Twenty miles from the base of the range, the slope is scarcely per- 

 ceptible, and only within short distances of the foot-hills is the fall in any 

 place very marked. 



Between the mouth of Big Thompson Canon, where the stream 

 leaves the mountains, and the town of Evans, near the junction of the 

 creek with the South Platte, there is a fall of about 600 feet in 20 miles, 

 an average of 30 feet per mile, which represents very closely the slope of 

 the plain in Northern Colorado. 



In the region of Horse Creek, in Wyoming, 80 miles to the northward, 

 and opposite the central portion of the Laramie Hills, the slope is shown, by 

 barometric measurements, to fall off in 22 miles, 775 feet, or an average of 35 

 feet per mile. Taking the grade of the Union Pacific Railroad, which may 

 fairly be regarded as indicating the rise of the land toward the mountains, 

 it shows between Sydney, in Nebraska, and Cheyenne, a distance of 100 

 miles, an increase in elevation of nearly 2,000 feet, or 20 feet per mile. 



