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Eureka Quartzite
The Eureka Quartzite is an extensive Paleozoic marine sandstone deposit in western North America that is notable for its great extent, extreme purity, consistently fine grain size, and its tendency to form conspicuous white cliffs visible from afar. The Eureka is commonly underlain and overlain by contrasting slope-forming limestone and dolomite formations, all of Ordovician age. It was named in 1883 for the Eureka mineral district in Nevada,〔Hague, Arnold (1883). ''Geology of the Eureka Mining District, Nevada''. U.S. Geological Survey, Third Annual Report. pp. 237–290.
〕 and that name is used almost exclusively in Nevada, but, in ensuing years, as extensions of the deposit were discovered in other areas, the same formation was given many other local names.
==Extent==
By whatever name, the Eureka can be traced, with gaps, northward through Idaho into western Canada along the British Columbia-Alberta boundary,〔〔 and southward to southeastern California,〔Ketner, Keith B. (1968). ''Origin of Ordovician quartzite in the Cordilleran miogeosyncline''. U.S. Geological Survey, Professional Paper 600-B. pp. 169–177.〕〔Ross, R.J., Jr. (1964a). ''Middle and Lower Ordovician formations in southernmost Nevada and adjacent California''. U.S. Geological Survey, Bulletin 1180-C. pp. C1–C101.〕 a north-south extent of about 2,200 km (1,400 mi). An isolated exposure was identified in Sonora, Mexico 950 km (590 mi) south of its most southerly exposure in California, but that occurrence probably was tectonically displaced there from California.〔Ketner, Keith B (1986). ''Eureka Quartzite in Mexico?--Tectonic implications''. Geology 14: 1027–1030〕 The Eureka and its correlatives are lenticular in cross-section: in Nevada and Utah the formation extends more than 300 km (190 mi) east-west, thinning out in both directions from maxima along its axis of more than 150 m (490 ft). In Canada the east-west extent is much less but there also, it thins out to both east and west.〔Norford, B.S. (1966). ''Ordovician-Silurian of the Cordillera in Geological History of Western Canada''. Alberta Society of Petroleum Geologists, Canadian Sedimentary Basins Symposium, Chapter 4, part 2. pp. 42–48.〕
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