Island Mountain is a resistant formation of volcanic and metavolcanic rock in the Franciscan Assemblage of southwestern Trinity County, California. It was mined for sulfur metal deposits during the first half of the 20th century and was the final obstacle in the construction of a railroad line to Eureka, California in 1914. The name is applied to the range, the peak, a pop…Island Mountain is a resistant formation of volcanic and metavolcanic rock in the Franciscan Assemblage of southwestern Trinity County, California. It was mined for sulfur metal deposits during the first half of the 20th century and was the final obstacle in the construction of a railroad line to Eureka, California in 1914. The name is applied to the range, the peak, a populated place and an abandoned railroad station, tunnel and bridge all but the first located within the S-bend of the Eel River which gave the peak its name.