Ball — Pre-Cambrian Rocks of Georgetown, Col. 389 



basic rocks. The granites are characteristically biotite-granites 

 in which microcline, which undoubtedly is in some cases a 

 product of recrystallization and in others of deformation, is 

 usually the predominant feldspar. Orthoclase in turn predomi- 

 nates over oligoclase or oligoclase-albite. Microcline through- 

 out the series is characterized by microperthite bands which 

 are less common in orthoclase. The presence in quartz of 

 abundant thread-like inclusions of undetermined uature is also 

 characteristic. Biotite appears to contain titanium, since rutile 

 or some other titanium mineral is a by-product of its altera- 

 tion. Original muscovite, except in certain of the pegmatitic 

 granites, is present in negligible amounts. Of the accessory 

 minerals, apatite, zircon and magnetite are constantly and 

 rather abundantly present. The granites of the Georgetown 

 quadrangle present a rather interesting example of the 

 repeated injection of a granitic magma or magmas of rather 

 constant composition. Mr. E. B. Mathews'" emphasized the 

 close chemical and mineralogical similarity of the granites of 

 the Pike's Peak (Colorado) quadrangle, which lies at the south 

 end of the Colorado Bange. The granites of this region and 

 those of the Georgetown quadrangle possess many character- 

 istics in common. 



* Journal of Geology, vol. viii, pp. 214-240. 



