Named for occurrences of quartz monzonite on Mount Princeton, Chaffee Co, CO in the Eagle basin. Geologic map. Exposed over large area on west and northeast sides of map in both Chaffee Co, CO in the Eagle basin and in Gunnison Co, Co in the Piceance basin. Exposures of lesser extent on east side Tomichi Creek. These occurrences are part of a large batholith, stocks, and apophyses and dikes. Total area of exposure of the batholith is probably not less than 200 sq mi. Is uniform in texture, medium- to coarse-grain with pink to white feldspar, quartz, biotite, hornblende, and accessory titanite. Quartz is subordinate. Is very weathered usually to white. Hornblende and biotite present in nearly equal amounts. Plagioclase is in excess of orthoclase. Has a border facies which is porphyritic with phenocrysts of pink orthoclase, white plagioclase, and biotite. Groundmass is intergrown quartz and orthoclase. Has inclusions of Paleozoic rock and of diorite. Intrudes youngest sediments in region; hence is younger than Pennsylvanian. Is older than post-Carboniferous granite, and later than Pomeroy quartz monzonite. Post-Carboniferous, probably Tertiary age.

Source: GNU records (USGS DDS-6; Denver GNULEX).