Ball — Pre- Cambrian Rocks of Georgetown, Col. 371 



Akt. XXXIY. — Pre-Camhrian Rocks of the Georgetown 

 Quadrangle, Colorado ;* by Sydney PL Ball. 



Introduction. 



In the summer of 1904 a geological survey of the George- 

 town (Colo.) quadrangle was made by the writer under the 

 supervision of Mr. J. E. Spun* and with the assistance of Mr. 

 O. H. Hershey. To each the writer gratefully acknowledges 

 great indebtedness. The present paper is a preliminary descrip- 

 tion of the pre-Cambrian formations exposed in this region. 



The Georgetown quadrangle is situated in the center of the 

 north half of Colorado and lies between the meridians 105° 30' 

 and 105° 45' west longitude, and the parallels 39° 30' and 

 39° 45' north latitude. The northeast corner of the quadrangle 

 is 26*5 miles west of Denver. Idaho Springs and Georgetown 

 are the principal towns. 



Topography . 



Relief. — Situated upon the east slope of the Colorado or 

 Front Range, the region is one of high elevation and of great 

 relief. Massive Mount Evans (14,260 feet) is the highest peak 

 in the quadrangle and is the center from which the main ridges 

 radiate. The lowest elevation is 7,450 feet, in the valley of 

 Clear Creek at the eastern edge of the quadrangle. 



Drainage.— The streams are all mountain torrents of steep 

 grade. The northern and central portions of the quadrangle are 

 drained by tributaries of the South Platte River, including 

 Clear Creek, its affluents, and the headwaters of Bear Creek. 

 The southern portion of the quadrangle is drained by tributa- 

 ries of the North Fork of the South Platte. 



Evolution of topographic forms. — From a point command- 

 ing a wide view of this portion of the Colorado Range, three 

 distinct topographic forms are recognizable ; first, an ancient, 

 mature mountainous upland ; second, Y-shaped valleys incised 

 in this upland ; and third, glacial cirques developed at the 

 heads of some of the valleys, passing below into U-shaped 

 valleys. 



The mountainous upland over considerable areas in the east 

 central and southeastern portions of the quadrangle has been 

 but little modified by recent erosion, and remnants of the old 

 surface are preserved on the crests of ridges throughout the 

 quadrangle. The mountainous upland was an ancient land 

 surface with about the same differences in altitude as those of 

 the present, surface. Dome-shaped mountains and smooth 



* Published by permission of the Director of the IT. S. Geological Survey. 



Am. Jour. Sci.— Fourth Series, Vol. XXI, No. 125.— May, 1906. 

 26 



