314 REPORT UNITED STATES GEOLOGICAL SURVEY. 



having the normal Eocky Mountain trend, that is, 2€° or 30<^ east of 

 south and west of north. The courses of the streams belong to one of 

 three systems: 1st. Tliose flowing nearly east or west. These are, in all 

 cases, tbe larger streams, and their courses, utterly disregarding, as they 

 do, tbe trend of the ranges, indicate that they preceded the uplift of the 

 latter, and their directions show the prevailing slopes of the land in those 

 earlier times. Rivers live longer than mountains. The courses of this 

 class of streams, known as " antecedent drainage," being in most cases 

 transverse to the ridges and valleys, we find them alternately and for 

 short distances only, in open valley and in close caiion. Of this class 

 are the two forks of the Platte, the Arkansas, the San Juan, the Gun- 

 nison, Grand, White, Yampa, and perhaps the Eio Grande. The 

 extreme heads of these streams belong plainly, however, to the second 

 class. This comprises all streams whose courses are parallel to the 

 ranges and are direct results of their uplift. The class includes most 

 second-rate streams, nearly all of which have courses parallel to the 

 ranges. Their courses are iu valleys of more or less width, seldom in 

 caiion. This class is known as " consequent drainage." 



The third class, the " superimposed drainage," comprises all streams 

 flowing down the slopes of the ranges, with courses, generally speaking, 

 at right angles to their trend. 



In this State these three classes of water-courses are very strongly 

 marked, and do not merge into one another, except in the case of 

 antecedent streams heading in consequent streams, which is, of course, 

 to be expected. An antecedent stream does not, in any case, become a 

 consequent stream farther down its course. The greater i)art by far 

 of the agricultural land in the mountain area of Colorado is in the val- 

 leys of consequent streams. 



Rising quite abruptly from the western border of the plains is a suc- 

 cession of mountain ranges placed en echelon, so as to present an almost 

 unbroken front to the plains from the northern to the southern bound- 

 ary of the State. The northernmost of these ranges extends from near 

 Long's Peak northward into Wyoming. It is known as the Laramie 

 Range, or the Black Hills of Wyoming. South of this is the Colorado or 

 Front Range, which overlooks the plains from Long's Peak southward to 

 the debouchure of the Arkansas from the mountains, and farther south- 

 ward the front rank of the Rocky Mountains is formed by the Sangre de 

 Cristo Range, with, for a short distance, the Wet or Greenhorn Mount- 

 ains as a skirmish-line. West of this chain of ranges lies a succession 

 (from north to south) of high, broad mountain valleys, separated from 

 one another by groups of eruptive mountains or mases of hills. These 

 valleys are known as the North, Middle, South, and San Luis Parks. 

 West of these valleys is another succession of ranges. The most north- 

 ern of these, the Park Range, forms the western wall of the three north- 

 ernmost of the parks ; and, west of North and Middle Parks, it is the last 

 range, the country to the westward consisting of a succession of broken 

 plateaus, gradually lessening in elevation as we trace them westward. 



West of South Park and the Park Range lies the consequent valley 

 of the Upper Arkansas, and around the southern end of this range, 

 through the broken hills which succeed it, the river finds its way to the 

 plains. Beyond the Valley of the Upper Arkansas rises the Sawatch 

 Range, and this is succeeded by the parallel ranges and irregular masses 

 of the Elk Mountains. 



The San Luis Valley is limited on the west by the great mass of the 

 San Juan Mountains, an immense, rugged nucleus, from which radiate 

 in all directions heavy, long spurs, many of which are of sufficient im- 

 portance to merit the name of ranges. 



