REPORT ON THE GEOLOGY OF A PORTION OF NORTHWESTERN 



COLORADO. 



CHAPTER I. 



INTRODUCTIOK 



The district upon which the following report is made, is iucluded 

 within the followiog boundaries: The eastern boundary is approxi- 

 mately upon a straight line drawn from a point where the meridian 

 of longitude 107o 50' west from Greenwich crosses White Eiver, to 

 where the meridian of 107° 25' crosses Yampa Eiver. The northern 

 boundary is the parallel of north latitude 40° 30' ; the southern is the 

 channel of White Eiver, and the western the meridian of 109° 30'. The 

 area thus inclosed- contains about 2,400 square miles. The Yampa 

 Eiver has its course near, and its general direction approximately coin- 

 cident with the northern boundary just mentioned ; bending and mean- 

 dering on each side of it, so that, in general terms, the Yampa and White 

 Eivers maybe said to be respectively the northern and southern bound- 

 aries of the district. The meridian which constitutes the western 

 boundary is thirty minutes west of the joint boundary-line between 

 Colorado and Utah. The eastern boundary is not a distinctly defined 

 one, because this district adjoins the one which was surveyed by the 

 late Mr. A. E. Marvine, and which was bounded on the west by no 

 definite geographical features. It is, however, practically as above 

 'stated. 



The Yampa and White Eivers are both tributaries of Green Eiver, 

 which is itself the principal tributary of the great Colorado Eiver of the 

 West. This district is thus shown to be upon the Pacific-drainage slope 

 of the continent, the great Eocky Mountain chain lying wholly to the 

 east of it, but some of the western foot hills of which rest upon its 

 eastern border. 



Strictly speaking, this district is only in part a mountainous one, 

 although it is by no means a plain-country in any considerable part. It 

 lies between two separate and important mountain systems, namely 

 those of the Uinta and the Park Eanges (the latter being a subordinate 

 portion of the great Eocky Mountain Eauge), each of which, especially 

 the former, occupies a portion of its area. The Uinta Mountains have 

 an east and west, and the Park Mountains a north and south trend, 

 or, in other words, their axes are approximatelj" at right angles with 

 each other. The eastern end of the Uinta Eange occupies a large portion 

 of the northwestern part of the district, while the eastern portion of 

 the district extends up to the western foot-hills of the Park Eange. 



The district is an elevated one, aside from its mountains. The lowest 

 level within its limits, which is the surface of Green Eiver, is about 

 5,000 feet above the level of the sea, while the higher points of land, 

 both those of the eastern and northwestern portions of the district, reach 

 an altitude of more than 9,000 feet above the level of the sea. In con- 

 se(j[uence of this great elevation and of the dryness of the climate, vege- 



5 



