The country rocks at this site are flyschlike metasedimentary rocks that gradationally intertongue with andesitic and basaltic metatuff (Berg and others, 1988, p. 17-19). The strata were regionally metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time (Brew, 1996, p. 27). Their premetamorphic age is uncertain. Berg and others (1988, p. 17) state that they closely resemble Upper Jurassic to mid-Cretaceous marine flysch and volcanic rocks nearby on Gravina Island. At the prospect, the bedded rocks are intruded by a 600-1000-foot-thick dike of granite porphyry (Wright and Wright, 1908, p. 156) or granodiorite (Maas and others, 1995, fig. 46). The dike probably is Late Cretaceous or younger, assuming that it postdates the regional metamorphism.

The deposit consists of a 3- to 3.5-foot-thick quartz fissure vein in granite porphyry (Brooks, 1902, p. 58; Wright and Wright, 1908, p. 156). The vein strikes northwest and has been traced in outcrop for 300-500 feet; it contains small amounts of pyrite, sphalerite, and galena, and minute particles of native gold. The prospect was developed in the early 1900s by an opencut and a 105-foot tunnel. Maas and others (1995, table 25) report the following average metal contents in their samples from the Rainy Day prospect: 6.28 ppm Au, 13.2 ppm Ag, 48 ppm Cu, 732 ppm Pb, and 14 ppm Zn.