Ball — Pre- Cambrian Rocks of Georgetown, Col. 379 



smaller areas occur east of Georgetown and west of Idaho 

 Springs. 



Petrography. — The gneissoid-granite is a fine to even- 

 grained granite, more or less gneissoid, which is gray when 

 fresh, and flesh pink to yellowish brown when weathered. 

 Feldspar, quartz and biotite are visible to the naked eye, 

 although the last is sometimes practically lacking. Muscovite 

 plates, which reach a maximum diameter of one-half of an inch 

 and enclose the other constituents poikilitically, are locally 

 prominent. Magnetite, pink garnet, and sillimanite are occa- 

 sionally visible. 



The quartz under the microscope is of the normal granitic 

 type except for the abundant inclusion in many of the grains 

 of needle-like opaque microliter. Orthoclase and micro- 

 cline greatly predominate over oligoclase or a related acid 

 plagioclase. Micropegmatitic intergrowths of each of the 

 feldspar species with quartz occur. The content of microcline, 

 often microperthitic, increases with increase in the recrystal- 

 lization to which the rock has been subjected, and, in conse- 

 quence, is absent in some slides and is the predominant feldspar 

 in others. While some microcline may be original, the larger 

 portion is certainly of secondary origin. It occurs in wedges 

 and hook like masses which sometimes separate quartz or ortho- 

 clase fragments of similar orientation ; again, elongated areas 

 are arranged end to end as if forced into planes of weakness. 

 In other cases altered plagioclase or orthoclase grains are dot- 

 ted by fresh areas of microcline similarly oriented and elongated 

 parallel to the cleavage of the orthoclase or the albite twinning 

 of the plagioclase. The host is often fractured and unshattered 

 microcline bridges the crack, while a narrow rim of orthoclase 

 or plagioclase next the microcline is fresh, in contradistinction 

 to the altered'feldspar of the main mass. While clearly later 

 than the other feldspars, the time at which this microcline 

 formed is unknown. 



Biotite has no unusual features. The poikilitic muscovite 

 plates, already mentioned, grade into sericitic shreds, the altera- 

 tion product of the feldspar, and are themselves secondary. 

 Sillimanite, which has also been previously mentioned, occurs 

 in parallel aggregates in the center of muscovite plates and 

 may be one of its alteration products. The accessory minerals 

 constantly present are zircon, apatite and magnetite, while 

 ihnenite and garnet sometimes occur. 



The granite magma when injected must have been very 

 fluid since it inserted itself between the folia of the earlier 

 gneisses and replaced in a marked degree inclusions of the 

 Idaho Springs formation. Dark bands in scroll-like patterns 

 of more basic granite preserve the outlines of schist inclusions, 



