in 1978. GEOLOGIC BACKGROUND Precambrian miogeosynclinal metasedimentary rocks were first recognized in the Medicine Bow Mountains by Hague (Hague and Emmons, 1877), who gave a relatively good description of these rocks, located in the north-central Medicine Bow Mountains. They were later described in more detail by Van Hise (Van Hise and Leith, 1909) and Blackwelder (1926). Blackwelder (1926) gave the first complete description of these rocks and named various units in the upper part of the succession. Of particular interest is the fact that in a later paper Blackwelder (1935) noted that these miogeosynclinal rocks strongly resembled the Huronian rocks of the Great Lakes region. The first description of miogeosynclinal metasedimentary rocks in the Sierra Madre was by Spencer (1904), who made a superb reconnaissance study of almost the entire area.