Ball — Pre- Cambrian Rocks of Georgetown, Col. 377 



elongation of the ragged hornblende prisms, and to a some- 

 what imperfect orientation of the hornblende cleavage. Horn- 

 blende and biotite together form over one-half the rock, while 

 plagioclase, usually labradorite, and smaller amounts of ortho- 

 clase and quartz are the other essential constituents. Titanite, 

 apatite, zircon and ilmenite or magnetite are accessories, and 

 augite, epidote and pyrite are sometimes present. The more 

 massive facies show traces of igneous texture, one rock from 

 the west bank of Soda Creek, .8 of a mile south of Idaho 

 Springs, being a metadiabase of ophitic texture in which the 

 ferromagnesian mineral is hornblende. 



Age. — The hornblende-gneiss cuts the Idaho Springs forma- 

 tion and is therefore younger. The gneiss itself is cut by the 

 other igneous rocks of the quadrangle. From the excellent 

 schistosity developed over wide areas in this much meta- 

 morphosed ba«ic igneous rock, it is probably almost as ancient 

 as the Idaho Springs formation. 



Quartz- Monzonite- Gneiss. 



Distribution. — The quartz-monzonite-gneiss is widely dis- 

 tributed, particularly in the central and southern portions of 

 the quadrangle. The largest area is on Paines Mountain. 



Petrography. — The quartz-monzonite-gneiss is a gray, 

 medium-grained gneissic rock which is normally porphyritic. 

 The gneissic structure is due to the segregation in alternating- 

 bands of quartz and feldspar and of biotite and hornblende. 

 The parting parallel to the gneissic structure is seen, under the 

 microscope, to be due to the parallel orientation of biotite blades 

 through recrystallization and to a slight elongation of the horn- 

 blende individuals parallel to the schistosity. The orientation 

 of the hornblende is probably largely due to the more vigorous 

 attack by solutions upon those hornblendes of the monzonite at 

 right angles or highly inclined to the plane of developing 

 schistosity than those whose elongation was originally parallel 

 to it. Apatite, zircon and titanite in certain instances show 

 marked parallelism to the gneissic structure, indicating the 

 extreme recrystallization to which the rock has been subjected. 



Rudely ellipsoidal white porphyritic striated feldspar crystals 

 lie in the medium-grained schistose aggregate of biotite, feld- 

 spar, hornblende and quartz. These feldspars are aligned 

 parallel to the gneissic structure and have suffered the same 

 deformation as the smaller constituents of the rock. The 

 quartz-monzonite-gneiss near West Geneva Creek contains also 

 pink microcline phenocrysts, sometimes 2 inches long', often 

 twinned according to the Carlsbad law. These have perfect 

 crystal outlines with their longest axis at right angles to the 

 gneissic structure even in the most metamorphosed groundmass. 



