Ball — Pre- Cambrian Hocks of Georgetown, Col. 383 



Rosalie Granite [Biotite- Granite). 



Distribution and Name. — Exposures of the Rosalie granite 

 are confined to the southern portion of the Georgetown quad- 

 rangle. The type locality is that in the southeast corner of 

 the quadrangle on Deer and Elk creeks, where the granite 

 weathers into dome-shaped hills and gentle valleys, covered by 

 mushroom-like forms of granite. A second area forms the 

 ridge between Mount Evans and Mount Rosalie and the granite 

 is named from the latter peak. 



Petrography. — The Rosalie granite is a coarse grained, mas- 

 sive, granular rock whose predominant constituent is a salmon- 

 pink microline often showing Carlsbad twinning. These feld- 

 spars, which vary in length from one-half to two and one-half 

 inches, are rudely tabular in form and are separated from one 

 another by ramifying bands of quartz, feldspar, biotite and 

 magnetite, all of medium size. 



Under the microscope the texture is uneven and hypidio- 

 morphic granular. The essential constituents of the rock 

 began to separate in the order usual in granites, but the periods 

 of separation of all overlapped, and in consequence each con- 

 stituent encloses blebs and partial crystals of the others. Mag- 

 netite in some cases separated simultaneously with quartz and 

 feldspar. The minerals in the order of their abundance are 

 microcline, quartz, orthoclase, oligoclase and biotite. Micro- 

 cline, often microperthitic, contains hexagonal plates, dots and 

 rods of hematite. These interpositions also occur frequently 

 in plagioclase and less frequently in quartz, and in association 

 with them in the latter mineral are opaque hair-like inclusions. 

 Quartz at its contact with the various feldspar species and 

 biotite forms micropegmatitic intergrowths. Zircon, apatite 

 and magnetite are constant accessory minerals, while muscovite, 

 titanite and pyrite are less frequent. With the exception of 

 minor granulation, the Rosalie granite is but little deformed. 



Age. — The Rosalie granite cuts the quartz-monzonite and in 

 turn is intruded by dikes of apparently the Silver Flume 

 granite described below. 



Silver Plume Granite {Biotite- Granite). 



Distribution and Name. — Stocks, dikes and irregular intru- 

 sive masses of the Silver Plume granite are especially well 

 developed in the vicinity of Georgetown, north of Meridian 

 Hill and on Alps Mountain, but occur widely distributed 

 throughout the quadrangle except in the extreme northeast 

 and southeast corners. It forms the south wall of the Clear 

 Creek canyon at the mining town of Silver Plume and it 

 derives its name from this village. 



