4 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



boundaries of the State, beyond which at either end it becomes gradually 

 lower, and disappears as a topographical feature beneath the plains. To 

 the west of this range lie the mountain valleys of the North, Middle, South, 

 and San Luis Parks, in Colorado, and the Laramie Plains, in Wyoming, 

 each of which possesses the same general feature of being nearly completely 

 encircled by mountain ridges. On the other hand, each has distinct topo- 

 graphical features of its own, which need not be entered upon here. 



Beyond the parks on the west, and separating them from the great 

 basin of the Colorado River, is a second mountain uplift, to which the gen- 

 eral name of Park Range has been given. It has b\' no means the regular 

 structure of the Colorado Range, but is made up of a series of short ranges 

 en dchelon, from which offshoots connect with the latter, forming the ridges 

 Avhich separate the different park basins. In the latitude of Leadville this 

 western uplift consists of two distinct ranges, the Mosquito or Park Range — 

 the latter being the name given in the Hayden atlas of 1877, pi-obably 

 because it forms the boundary of the South Park — and the Sawatch Range, 

 which forms the water-shed between the Atlantic and Pacific waters. 



The Mosquito Range is a narrow, straight ridge, about eighty miles in 

 length, trending a little west of north, and is characterized by long, regular 

 slopes scored deeply by glacial gorges on the east toward South Park and 

 bv an abrupt irregular inclination on the west towards the Arkansas Valley. 



The Sawatch Range, on the other hand, is a broader, oval-shaped 

 mountain mass, divided by the deep gorges of its draining streams into a 

 series of massives and wanting the continuous ridge structure of the Mos- 

 quito Range. In tliis respect, as in its geological composition, which is the 

 determining cause of the difference of its topographical forms, it resembles 

 the Colorado Range. The culminating points of each range have a remark- 

 ably uniform elevation of about fourteen thousand feet above sea-level. 



Between the two ranges lies the valley of the Upper Arkansas, a merid- 

 ional depression 60 miles in length and about sixteen miles in width, measured 

 from the crest of its bounding ridges. Its direction is parallel to that of 

 the Mosquito Range, being a little east of south in its mean course, though 

 more nearly north and south towards its head. From its southern end the 

 Arkansas River, after receiving the waters of the South Arkansas, bends 



