THE MUSQUITO EANGE. 27 



thev (iccu|)\', certainly in the influence wliicli they have had upon the con- 

 centration of mineral formation. 



In that i)ortion of the Rocky Mountain region under consideration there 

 is a noticeable connection between the structural lines and those along which 

 eruptive action has been most active. The latter correspond witli the lines 

 of weakness, of greatest folding and faulting. Leaving out of consideration 

 the dikes which traverse the Archean rocks, which, though numerous, are 

 of relativelv small mass, the eastern uplift gives evidence of little eruptive 

 activity, it being shown only liy a few isolated outflows of Tertiary lavas. 

 Along the line of the parks, on the other hand, both earlier and later erup- 

 tions are so frequent that theii' outcroj)s form an almost continuous line from 

 north to south parallel witli the western uplift, while along the west base of 

 the latter the Elk Mountains, the head of White River, and the Elk Head 

 Mountains in Wyoming have apparentl}" been the scenes of most violent and 

 repeated eruptions during both Mesozoic and Tertiary times. 



MOSQUITO RANGE. 



Topography. — That portion of the Mosquito Range the study of whose 

 geological structure was considered necessary for a ]iroper comprehension 

 of the ore deposits of Leadville is shown in relief on Atlas Sheet V. It 

 comprises a length of 19 miles along the crest of the range, and in ^\idth 

 includes its foot-hills, bordering the Arkansas Valley on the west and South 

 Park on the east, a slope in the one case of seven and one-half miles and 

 in the other of about nine miles in a direct line. This is essentially an 

 alpine region, scarcel)^ a point within the area of the map being less than 

 10,000 feet above sea level. 



In this area the range has a sharp single crest trending almost due 

 north and south, the echelon structure being, however, developed on the 

 northern and southern limits of the map respectively. To the west this 

 crest presents abrupt escarpments, descending precipitously into the great 

 glacial amphitheaters which exist at the head of almost all the larger streams 

 flowing from the range. The spurs have extremely irregular, jagged out- 

 lines, resulting from the numerous minor hills which rise above the average 

 slope. Within a few miles of the valley bottom, however, tlieir form sud- 



