McNulty rhyolite is one of four intrusive, igneous geological formations, the Chalk Mountain nevadite, Lincoln porphyry, McNulty rhyolite and Quail porphyry, described, mapped, and named by S. F. Emmons[1] in 1898 within the Tenmile Mining District of southern Summit County, Colorado.
One exposure above the Railroad Boy tunnel, his location 45 in McNulty Gulch, exhibited small drusy cavities containing little tablets of tridymite.
He proposed that this rhyolite was either intruded contemporaneously withy or later than the Chalk Mountain nevadite at the time of eruption.
Based on field mapping, the McNulty rhyolite was interpreted to cross-cut and post-date the Lincoln and the Quail porphyries.
[8] Within the Tenmile Mining district, the primary ore horizons do not lie within the unnamed quartz monzonite porphyry, which includes the McNulty rhyolite.