The country rocks in this part of Revillagigedo Island are marine, interbedded, andesitic and basaltic metavolcanic rocks and subordinate pelitic metasedimentary rocks that are intruded by stocks, sills, and dikes of Cretaceous feldspar-porphyritic granodiorite (Berg and others, 1988). The strata and some of the granodiorite were regionally metamorphosed to greenschist grade in Late Cretaceous time. These regionally metamorphosed rocks subsequently were locally remetamorphosed to hornblende hornfels near the contacts of Cretaceous granodiorite plutons that were emplaced after the regional metamorphism. The premetamorphic age of the strata is uncertain. Berg and others (1988) assign them a Mesozoic or (late) Paleozoic age. Berg (1982) and Crawford and others (2000) assign them to the Gravina belt, of Late Jurassic or Cretaceous age, or to the Taku terrane, of late Paleozoic to Late Triassic age. The metamorphic and intrusive rocks locally are overlain by basalt and andesite lava flows of Quaternary or Tertiary age.

According to Wright and Wright (1908, p. 148), the Baltic prospect (which they erroneously called 'Baltic Star') consists of a 1.5-foot-thick quartz fissure vein in mineralized schist. The vein, which was traced on the surface for 300 feet, strikes NE and dips 75 SE, and contains small amounts of pyrite, sphalerite, and galena, in addition to low values in gold. The northeast end of the Baltic claim abuts against a Quaternary basalt lava flow.