376 Ball — Pre-Cambrian Pocks of Georgetown, Col. 



series, while the bands containing the ellipsoidal masses are 

 most naturally regarded as conglomerates. The quartz-gneiss 

 is present in such thick masses that it can scarcely be of 

 pegmatitic or vein origin and probably represents intensely 

 metamorphosed sandstone. The silicate rocks would in this 

 interpretation of the origin of the formation represent cal- 

 careous sandstones and impure limestones, while the biotite-sil- 

 limanite-schist and biotite-schist would be metamorphosed 

 shales and arkoses. The abundance of aluminum silicates in 

 the series and the similarity to metamorphic rocks of known 

 sedimentary origin strengthens the view of the supposed sedi- 

 mentary origin of the Idaho Springs formation. This forma- 

 tion then may be regarded as an intensely metamorphosed series 

 of shales, arkoses, sandstones, conglomerates and impure lime- 

 stones. 



Age. — The Idaho Springs formation is the oldest member 

 of the pre-Cambrian series in the Georgetown quadrangle, 

 and it forms the network into which the other formations 

 were injected. The pre-Cambrian quartzite of South Boulder 

 Creek (Colorado), described by Dr. C. R. Yan Hise,* lies 

 unconformably upon a granite which in the amount of mash- 

 ing suffered and in lithological character somewhat resembles 

 the Silver Plume granite later to be described. While the 

 gap of IS miles between Idaho Springs and South Boulder 

 Creek has not been traversed, it is believed that the Idaho 

 Springs formation is vastly older than Yan Hise's pre-Cam- 

 brian quartzite. 



Hornblende- Gneiss. 



Distribution. — The hornblende gneiss has a wide distribu- 

 tion, especially in the southern portion of the quadrangle. It 

 occurs in sheets and dikes in the Idaho Springs formation and 

 may have formed surface flows. 



Petrography. — The hornblende-gneiss is typically a rather 

 fine-grained, well-banded rock in which white laminae of 

 quartz and feldspar alternate with black or greenish-black 

 layers of hornblende. Biotite is developed at the expense of 

 hornblende where maximum movement has occurred, and 

 along shear planes the rock passes to a biotite-hornblende- 

 schist. Aggregates of hornblende or biotite, or both, give a 

 porphyritic aspect to certain facies. Rather massive, fine- 

 grained facies occur but are rare. 



The microscope shows the banding to be partially a segrega- 

 tion into laminae of like minerals during recrystallization and 

 partially a result of lit-par-lit pegmatitic injections. The 

 parting parallel to the banding is largely due to the common 



*U. S. Geol. Surv. Bull. 86, p. 325. 



