112 DESCRIPTIVE GEOLOGY. 



SECTION IV. 



THE NORTH PARK. 



BY ARNOLD HAGUE. 



Physical Description. — Of the great areas of depression lying west 

 of the Colorado Range, known as the Parks, which form so characteristic 

 a physical feature of the Rocky Mountains, the North Park is the only 

 one that comes within the limits of this exploration, and, as its name would 

 suggest, occupies the most northerly position of these basins. 



The North Park is situated entirely within the Territory of Colorado; 

 its northern rim lying about 5 or 6 miles south of the forty-first parallel, the 

 boundary -line between Wyoming and Colorado. It lies depressed between 

 two great Archaean uplifts, the Medicine Bow and Park Ranges, measuring 

 at its widest expanse, from the base of the one to the foot-hills of the other, 

 between 28 and 30 miles, but narrowing toward the north. To the north- 

 ward, these two ranges come together; the former trending to the north- 

 west, and the latter stretching out eastward, so as to shut in the Park basin 

 on the north. In this way, the Park is enclosed on three sides by great 

 walls of rugged Archaean rock, with clear, sharply-defined limits. On the 

 south side, the boundary between the North and Middle Parks is by no 

 means so prominent or so regular a feature, and is formed by a compara- 

 tively low cross-ridge of Tertiary volcanic outbursts, which stretch the 

 entire width of the Park from Rabbit Ears in the Park Range to the 

 slopes of Mount Richthofen in the Medicine Bow. The topographical 

 divide of this southern ridge lies one or two miles beyond the limits of the 

 map, but the northern slopes are well shown, furnishing the evidences as 

 to both its physical and geological aspect. From north to south, the Park 

 measures about 35 miles, being roughly quadrangular in form, with its 

 meridional sides the longer. 



Some of the structural details of the Medicine Bow Range have already 

 been given, and those of the Park Range will be found under the proper 



