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



while lace-like shreds of biotite often suggest the forms of 

 original inclusions now almost totally absorbed. Similar phe- 

 nomena are referred to at greater length under the pegmatites 

 and associated granites and granite-porphyry. (See page 386.) 



The rock varies, often in the same exposure, from an almost 

 massive to a banded rock in which quartz and feldspar layers 

 are separated by discontinuous sheets of aligned biotite plates. 

 Microscopic examination shows that the gneissoid structure is 

 due largely to recrystallization and partially to granulation. 

 The more gneissoid facies occur about the border of large areas 

 of granite or around gneiss inclusions. 



Age. — The gneissoid-granite injects and encloses portions of 

 the formations already described and is, in turn, included in 

 and cut by the quartz-monzonite and the succeeding igneous 

 formations. 



Quartz- Monzon ■lie. 



Distribution. — A large irregularly shaped batholith of 

 quartz-monzonite, which disappears beneath the older rocks at 

 rather low angles, occupies the central, north central and east 

 central portions of the quadrangle. Minor intrusive masses are 

 rather widely distributed. 



Petrography. — The quartz-monzonite is a gray to bluish- 

 gray, medium-grained, granular rock, often more or less por- 

 phyritic in habit. Macroscopically, feldspar, quartz, biotite 

 and hornblende ars essential constituents, while magnetite and 

 titanite are accessories. In the porphyritic facies rather good 

 crystals of pink microcline feldspars, which are usually 

 twinned according to the Carlsbad law, reach a maximum 

 length of 1 inch. A matted coating of tiny epidote crystals 

 occurs rather characteristically along many joint fractures. 



The texture upon microscopic examination proves to be 

 hypidiomorphic granular, rather even-grained in the non-por- 

 phyritic and uneven in the porphyritic. With variation in 

 the relative amount of the alkali and lime-soda feldspars, the 

 rock varies from an acid to a basic quartz-monzonite with 

 granodioritic affinities. The essential minerals in the order of 

 their abundance are oligoclase or andesine, microcline often 

 microperthitic, quartz and biotite, and orthoclase and horn- 

 blende if present. The accessory minerals, magnetite, apatite, 

 pyrite and zircon, are particularly abundant. The order of 

 the solidification is normal, although the period of the separa- 

 tion of biotite overlapped that of titanite, and it in turn was 

 overlapped by that of plagioclase. Quartz is micropegmatiti- 

 cally intergrown with each of the feldspar species. Needle- 

 like opaque microlites are characteristic interpositions in quartz, 

 and magnetite cubelets and hematite in hexagonal plates occur 



