378 Ball — Pre-Canibrian Rocks of Georgetown, Col. 



Microscopic examination shows that they are imcracked while 

 the feldspars of the groundmass are fractured. They are with- 

 out much doubt of metamorphic origin and were formed after 

 the rock became a gneiss. In certain exposures these micro- 

 cline feldspars have been mashed by later deformation into 

 ellipsoids and even into ribbon-like masses. 



The original rock was a porphyritic quartz-monzonite, in 

 some facies of which the alkali feldspar is so subordinate that 

 .the rock becomes a granodiorite. Plagioclase is more abundant 

 than the other essential constituents, quartz, microcline, ortho- 

 clase, biotite and hornblende, and of these latter any one may 

 predominate. Hornblende, however, is not present in some 

 slides. The plagioclase in the "groundmass" and the ellipsoidal 

 porphyrite crystals is either oligoclase or andesine. The por- 

 phyritic plagioclase is characterized by zonally arranged mag- 

 netite cubelets, and by rods, hexagonal plates, and dots, proba- 

 bly of hematite. Micropegmatitic intergrowths of quartz and 

 each of the three feldspars occur along the borders of the 

 grains. The alterations of biotite are interesting in that when 

 hornblende is present epidote is the major alteration product, 

 while, on the other hand, when hornblende is absent, chlorite 

 or muscovite accompanied by rutile needles is the alteration 

 product. The rock is characterized by the unusual abundance 

 and coarseness of the accessory minerals, magnetite, titanite, 

 apatite, zircon and pyrite. Fyrite is usually surrounded by or 

 intergrown with magnetite. 



Age. — The quartz-monzonite-gneiss intrudes and includes 

 fragments of the Idaho Springs formation and the hornblende- 

 gneiss. It is in turn cut by the gneissoid-granite and the igneous 

 rocks which succeed it. While in some exposures its age rela- 

 tions with the Idaho Springs formation, the hornblende-gneiss 

 and the gneissoid-granite are clear, the quartz-monzonite-gneiss 

 usually contacts with these in a sharp line with parallel gneis- 

 sic structure. South of Naylor Lake some exposures occur in 

 which the Idaho Springs formation and the quartz-monzonite- 

 gneiss seem to grade into one another, apparently as the result 

 of the absorption of the schist hy the mbnzonite when injected. 



The close resemblance in mineral composition of the quartz- 

 monzonite-gneiss to the quartz-monzonite is striking, forming 

 a good example of the repetition of a monzonite injection. 



Gneissoid- Granite. 



Distribution. — Stocks and dikes of gneissoid-granite inject- 

 ing the formations already described are widely distributed in 

 the quadrangle. The gneissoid-granite covers large areas in 

 the southwestern corner of the quadrangle and numerous 



