Island Mountain is a resistant formation of

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

and metavolcanic rock in the

Franciscan Assemblage The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew ...

of southwestern Trinity County, California. It was mined for

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

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 Eureka (Wiyot: ''Jaroujiji'', Hupa: ''do'-wi-lotl-ding'', Karuk: ''uuth'') is the principal city and county seat of Humboldt County in the Redwood Empire region of California. The city is located on U.S. Route 101 on the shores of Humboldt ...

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.

Geography

Island Mountain is named peak in the Island Mountain range which covers an area from Mendocino County into Trinity County. The highest point of this range, and its official centroid, is a benchmark called "South Peak" on the unnamed tallest point which is in Mendocino County. Island Mountain in Trinity County was named by settlers in the 1850s because it is nearly isolated by water from two creeks and the Eel River. Due to the resistance of the rock to erosion, the Eel River makes an abrupt "S" curve around Island Mountain in its otherwise north-northwesterly flow between the

California Coast Ranges The Coast Ranges of California span from Del Norte or Humboldt County, California, south to Santa Barbara County. The other three coastal California mountain ranges are the Transverse Ranges, Peninsular Ranges and the Klamath Mountains. P ...

. It was noted as "Island Peak" by John Rockwell of the Coast Survey in 1878.

Railroad history

The Island Mountain itself () was the final obstacle to completion of the

Northwestern Pacific Railroad The Northwestern Pacific Railroad is a regional shortline railroad utilizing a stretch of the 271 mile mainline between Schellville and Windsor with freight and Sonoma–Marin Area Rail Transit (SMART) commuter trains. Formerly, it was a regi ...

between

San Francisco San Francisco (; Spanish language, Spanish for "Francis of Assisi, Saint Francis"), officially the City and County of San Francisco, is the commercial, financial, and cultural center of Northern California. The city proper is the List of Ca ...

and Eureka. Railroad construction proceeded northerly down the west side of the Eel River

canyon A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...

and bridged the river at the "S" curve to enter tunnel number 27 through the mountain. There was a golden spike ceremony downstream of the mountain at Cain Rock in 1914 to complete the railroad. The redwood framing of tunnel 27 burned on September 6, 1978, and a significant length of the tunnel collapsed. Tunnel rebuilding costing 3.8 million dollars was completed on December 7, 1979. Stop number 293, also named "Island Mountain" on the Northwestern Pacific Railroad was located along the tracks near the river where a railroad bridge crosses the Eel prior to the tracks entering the south end of Island Mountain tunnel at milepost 194.8.

Mining history

volcanogenic massive sulfide ore deposit Volcanogenic massive sulfide ore deposits, also known as VMS ore deposits, are a type of metal sulfide ore deposit, mainly copper-zinc which are associated with and created by volcanic-associated hydrothermal events in submarine environments. Th ...

within Island Mountain was mined after rail service became available. Between 1915 and 1930, 4,100 tons of

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

, 140,000 ounces of

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

, and 8,600 ounces of

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

were mined. Minerals identified within the ore body include

chalcanthite Chalcanthite (, ) is a richly colored blue-green water-soluble sulfate mineral . It is commonly found in the late-stage oxidation zones of copper deposits. Due to its ready solubility, chalcanthite is more common in arid regions. Chalcanthite i ...

, chalcopyrite, copiapite,

goslarite Goslarite is a hydrated zinc sulfate mineral () which was first found in the Rammelsberg mine, Goslar, Harz, Germany. It was described in 1847. Goslarite belongs to the epsomite Epsomite, Epsom salt, or magnesium sulfate heptahydrate, is a hyd ...

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

, and pyrrhotite. Volcanic rock was quarried from Island Mountain by Northwestern Pacific Railroad for heavy riprap used as far away as

Humboldt Bay Humboldt Bay is a natural bay and a multi-basin, bar-built coastal lagoon located on the rugged North Coast of California, entirely within Humboldt County, United States. It is the largest protected body of water on the West Coast between Sa ...

Populated place

Island Mountain is the name of a populated place located in Trinity County located at , above sea level. The first post office here was established in 1905 under the name "Island" until August 16, 1915 when it was renamed "Island Mountain."

References

External links

Island Mountain
from Western Mining History.
Images
of the tunnel and bridge. * * {{Eel River Mountains of Trinity County, California Mountains of Mendocino County, California History of Humboldt County, California Mountains of Northern California