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 in 1898 within the Tenmile Mining District of southern Summit County, Colorado. The McNulty rhyolite, which is also known as the McNulty Gulch rhyolite, is described by S. F. Emmons as a fine-grained porphyritic rhyolite that is light gray in color and contains many small white feldspars and locally some small smoky quartz crystals. He mapped it as being exposed as small irregular masses in McNulty Gulch and southward beyond the area of the Tenmile Mining District that was mapped at the time. One exposure above the Railroad Boy tunnel, his location 45 in McNulty Gulch, exhibited small drusy