Ball — Pre- Cambrian Rocks of Georgetoion, Col. 37 5 



The biotite-schist is differentiated from the type just 

 described by a finer grain, a less perfect schistosity (continuous 

 films of biotite being absent), by a medium or light gray color, 

 and by the almost total absence of muscovite and sillimanite. 

 Small segregations of magnetite surrounded by white halos 

 from which they have abstracted all the ferromagnesian min- 

 erals are rather common. 



The quartz-gneiss is a well banded, dense, vitreous rock 

 varying in color from gray to brown, red or black. Quartz 

 greatly predominates over all other constituents, which include 

 most of those present in the biotite-sillimanite-schist. Under 

 the microscope the gneiss is composed of intricately interlock- 

 ing quartz lenses elongated parallel to the banding, the darker 

 color of certain bands being due to magnetite cubes in discon- 

 tinuous rows. The quartz-gneiss is exposed for a distance of 

 one-half of a mile on Sugarloaf Peak. 



Distributed in bands in the three types of the Idaho Springs 

 formation already described, but particularly characteristic of 

 the biotite-schist, are white ellipsoidal masses from one-half to 

 four inches in length. These sharply bounded masses are 

 flattened parallel to the plane of schistosity and in some 

 instances are mashed to paper-thin sheets. They are composed 

 of quartz and sillimanite, other minerals being present only in 

 small amounts. The ellipsoidal masses occur widely over the 

 area, but are particularly well developed on Chief and Pendle- 

 ton mountains. 



The silicate rocks of the Idaho Springs formation, which 

 include many intergrading facies of widely varying composition 

 and texture, grade into the quartz-gneiss. The facies are horn- 

 blende-augite-feldspar-gneiss, quartz-magnetite-gneiss, and sev- 

 eral kinds of massive rocks. These massive rocks include 

 coarse-grained aggregates of quartz, epidote and brown garnet 

 in varying proportions with texture of a miarolitic pegmatite, 

 and lime-silicate rocks composed of dominant calcite with 

 smaller amounts of scapolite, grossular garnet, bottle-green 

 pyroxene and quartz. The microscope shows calcite to be 

 rather widely distributed and titanite to be a common accessory 

 mineral in the silicate rocks of the Idaho Springs formation. 

 Perhaps the most striking textural peculiarity is the presence 

 of micropegmatitic intergrowths of epidote and zoisite, epidote 

 and garnet and calcite and garnet, all clearly the products of 

 recrystallization. The silicate rocks are well exposed in the 

 vicinity of Alpine Peak. 



Origin of the Idaho Springs Formation. — The Idaho 

 Springs formation has been so greatly metamorphosed that all 

 original textures have been destroyed. The lithologic varia- 

 tion across apparently bedded bands suggests a sedimentary 



