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McNulty rhyolite
Found in a single location on the earth’s surface, McNulty rhyolite is a comparatively rare gem rock (geology) quality U.S. variety of rhyolite rock. McNulty rhyolite appears in the official U.S. Department of Interior, United States Geological Survey ''Lexicon of Geological Names of the United States''.〔''Lexicon of Geological Names of the United States (Including Alaska)'' Part 2 M-Z (Geological Survey Bulletin 896) Mary Grace Wilmarth, compiler, p. 1260, United States Dept. of Interior, Washington, D.C.: U.S. Government Printing Office (1938) (Univ. of Mich., Digitized February 18, 2010)〕
==Differentiated from common rhyolite==
Rhyolite is an igneous, volcanic rock of felsic. It often resembles granite and has the same basic mineral makeup; however, rhyolite’s origin is extrusive and not plutonic. Because it is so rich in silica, its melts or lavas, unlike those of basalt, are highly viscous. In volcanic extrusion, rhyolite tends not to flow, but, rather to plug expanding gases until thrown off in eruption in release of the pent up force of the expansion. In the U.S. today, rhyolites are most commonly found in the west and southwest. Unlike granite, some few varieties of rhyolite, like McNulty rhyolite, are coveted by lapidaries as gem rocks, because of their identifying mineral aggregates, interesting colors and flow banding. Other examples of varieties of rhyolite, suitable for use as gem rock (though both far, far more common than McNulty rhyolite), are “Wonderstone” and “Hichoryite”, which are together found in Mexico, Nevada, Utah and New Mexico.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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