THE MOSQUITO EANGE. 23 



sedimentary beds were deposited, which have since been so extensively 

 eroded off that the age or extent of these lakes cannot readily be determined. 



western uplift. The western boundary of the park area consisted of two 

 or more distinct ridges or islands, forming, however, a general line of eleva- 

 tion nearly parallel with that of the Colorado Range. These are the Park 

 Range proper, on the west side of the North Park, and the Sa watch Range, 

 now separated from the South Park by the Mosquito Range. Between these 

 was the Archean mass of the Gore Mountains, which formed, with the 

 southern extremity of the Park Range, the western wall of the Middle Park, 

 of whose geological relations but little is definitely known. 



The present topographical boundary of the South Park on the west is 

 the Mosquito Range, which has for this reason been also called the Park 

 Range. Geologically, however, this name is less appropriate than topo- 

 graphically, since prior to Cretaceous times no Mosquito Range existed, 

 but the rocks which now form its crest still rested at the bottom of the sea. 

 The Sawatch range forms the normal southern continuation of the Park 

 Range as an original Archean land-mass; hence it seems advisable to avoid 

 the use of the name Park Range in this latitude. 



The Archean land-mass of the Sawatch in Paleozoic times, judging from 

 the almost continuous fringe of Cambrian beds encircling it, as shown on 

 the Hayden maps, which may be assumed to represent a tolerable approxi- 

 mation to its original outlines, was an elliptical-shaped area, trending a little 

 west of north, with a length of about seventy-five miles and an extreme 

 breadth of about twenty miles. Through the eastern portion of this area, 

 and parallel with its longer axis, runs the valley of the Upper Arkansas 

 River, now an important feature in the topography, but which during 

 Paleozoic and Mesozoic times did not exist. 



The relative height of these mountain masses above the adjoining 

 valleys must have been far greater then than now, since the sedimentary 

 beds which surround them must have been formed out of the comminuted 

 material abraded from their slopes. It is probable, however, that they 

 were not the only land masses at that time, and future geological studies 

 in this region will doubtless decipher many yet unopened pages in its 

 past history. The great area of volcanic rocks to the southwest, whose 



