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Calcic hornblendes from Precambrian metamorphic and igneous terranes in the central Front Range, Colo., were subjected to a sillimanite-grade metamorphism and have compositions that are more dependent on host-rock mineral a~semblages than on host-rock chemical composition. The partitioning of the mafic oxides Mg, Fe+ 2 , Fe+ 3 , and Mn between biotite and hornblende is necessary in relating the total mineral assemblage to the composition of the host rocks. Orderly distribution of the mafic oxides between biotite and hornblende yields rather consistent distribution patterns, and these patterns appear to relate to the regional metamorphism. Fe+ 3 in the metamorphic host rocks controls, though perhaps imperfectly, the Mg content in hornblende, whereas Fe+ 3 in igneous host rocks only exerts partial control over Mg in accompanying hornblende. In general, oxidation-reduction ratios in the igneous hornblendes increase with increases in Fe+ 3 but decrease in metamorphic hornblendes with increasing Fe+ 3 ; FIGURE 1 (facing page). -Generalized geologic map of a part of the Front Range, Boulder, Gilpin, and Jefferson Counties, Colo.

In three lunar samples the high titanium contents previously reported were confirmed. Water and ferric oxide were absent.