388 Ball — Pre-Cambrian Rocks of Georgetown, Col, 



In the northern part of the quadrangle a white medium- 

 grained museovite-granite, which often has pink garnet as an 

 abundant accessory, grades into muscovite-pegmatite. North 

 of Georgetown the granitic rock is a gray muscovite-bearing 

 alaskite in which quartz predominates over feldspar. In the 

 central or southern parts of the quadrangle a pink or pinkish 

 gray fine-grained biotite-granite predominates. The same 

 granite in the southeast corner of the quadrangle is medium- 

 grained. ' 



Under the microscope the order of crystallization is normal 

 except that the rare accessory mineral titanite in certain 

 instances lies in irregular grains between quartz and feldspar 

 and therefore separated simultaneously with them. Micro- 

 cline, usually with microperthitic bands, is the predominant 

 constituent of most of the granites. Orthoclase and an acid 

 plagioclase, oligoclase-albite, are also usually present. Quartz 

 forms micropegmatitic in'tergrowths with the three species of 

 feldspars and is further characterized by abundant thread-like 

 inclusions. Biotite and muscovite are in no way peculiar 

 although original muscovite is more widely distributed than in 

 any other granite of the quadrangle. Zircon, magnetite and 

 apatite are common and abundant accessory minerals, while 

 titanite, garnet and fluorite sometimes occur. 



In the southeastern portion of the quadrangle pegmatite and 

 the associated granite grade into pink granite-porphyry. The 

 groundmass is a fine-grained microgranitic aggregate of micro- 

 cline and quartz, and in some facies orthoclase and magnetite 

 and in others biotite. The phenocrysts include pink tabular 

 microcline and orthoclase, rounded slightly smoky quartz, bio- 

 tite and sieve-like hornblende crystals. Titanite and magnetite 

 are accessory. Fluorite is a rather constant accessory of the 

 granite-porphyry and the associated granite. It forms trans- 

 parent colorless wedges, dotted by deep purple spots, between 

 quartz and feldspars, which in contact with it have slightly 

 rounded faces. Fluid inclusions are fairly abundant. Fluo- 

 rite occurs in an area ten miles distant from known mineral- 

 ized veins, and from this and the uniform distribution of the 

 mineral in the slides examined it is believed to have been 

 deposited in minute cavities in the rocks by pneumatolytic 

 action. 



Similarity of the Pre- Cambrian Granites. 



Although no chemical analyses of the rocks of the George- 

 town quadrangle have been made, microscopic examination 

 shows a marked mineralogic similarity in the granites, the 

 intrusions of which were separated in every case by consider- 

 able time intervals and in one case by the intrusion of more 



