ABSTRACT
A well-defined exposure of the Idaho Springs formation, interspersed with Silver Plume granite, is found at the southeast end or Hall Valley in the northeast section of Park County, Colorado. The Idaho Springs group of rocks of Precambrian age consists of quartz-biotite schist, quartz-biotite-sillimanite schist, biotite-actinolite gneiss, hornblende-diopside gneiss, hornblende-plagioclase gneiss, highly calcic lime-silicate rocks, and quartzites. They were formerly impure argillites, Magnesian and highly calcic
limestones, and sandstones which have been subjected to repeated regional metamorphism. In late pre-Cambrian time these rocks were intruded by a few mafic dike s and by the Silver Plume granite and its related pegmatites.
