Andesite, rhyolite, tuff, and generally siliceous volcaniclastic rocks make up the Koipato Group, which lies unconformably below the Humboldt assemblage. The Koipato Group consists of altered porphyritic andesite flows and flow breccia of the Limerick Greenstone, altered felsite and coarse-grained tuffaceous sedimentary rocks of the Rochester Rhyolite, and quartz-rich ash-flow tuff and tuffaceous sedimentary rocks of the Weaver Rhyolite. It is present in Churchill, Humboldt, Lander, and mostly Pershing Counties where it unconformably overlies deformed rocks of the Golconda terrane (GC). The upper part of the Koipato contains late Early Triassic (Spathian) fossils (Silberling, 1973; Wallace, Tatlock, and others, 1969). It is depositionally overlain by the Star Peak Group (TRc), a sequence of carbonate platform deposits at the base of the Humboldt assemblage. Radiometric dates from the 1970s (McKee and Burke, 1972) suggest a Middle to Early Triassic age.