Bulletin of the United States Geological Survey--Corundum and Its Occurrence and Distribution in the United States (A Revised and Enlarged Edition of Bulletin ) . dotite area, showing the relation of the amphibolitt dikes. spar is not constant in all of the amphibolites, but where it doesoccur it varies in size from minute particles to masses as large as a pea. The rocl^ has often a strikingly laminated structure, and gradesfrom this extreme to a structure showing no lamination at all. It isexceedingly tough and very fine grained. The corundum, which ioccurs in the amphibolite as an acce
Bulletin of the United States Geological Survey--Corundum and Its Occurrence and Distribution in the United States (A Revised and Enlarged Edition of Bulletin ) . dotite area, showing the relation of the amphibolitt dikes. spar is not constant in all of the amphibolites, but where it doesoccur it varies in size from minute particles to masses as large as a pea. The rocl^ has often a strikingly laminated structure, and gradesfrom this extreme to a structure showing no lamination at all. It isexceedingly tough and very fine grained. The corundum, which ioccurs in the amphibolite as an accessory mineral, varies in size fromminute particles to masses several inches in diameter, in which thereare usually developed parting planes parallel to the unit rhombohe-dron. In color it varies from almost white to a deep ruby-red, butthe prevailing color is a deep pink. Very rarely deep blue corundumhas been found in these amphibolites. On account of the exceeding toughness of the rock, and more par-ticularly on account of the low percentage of corundum, these am-phibolites are not of commercial value as a source of corundum; they CORUNDUM IN IGNEOUS ROCKS. 37. )H \ :^ do, however, make handsome mineral specimens. In fig. 10 there isilhistrated a mass of corundum in amphibolite from Buck Elf, Clay County, N. C, a similar occurrence of corundum inamphibolite has been observed Avhere this rock cuts a ty})ical dunite. At the Isbel mine, at the head of Shooting Creek, in Chiy County,N. C, corundum occurs irregularly, and is sparingly disseminatedthrough a decomposed hornblendic rock in small grains and least decayed portions of the rock seem to possess the charactersof an amphibolite, but this could not be determined with corundum occurs through the mass of this rock across an outcropabout 153 feet wide. In most of it the rhombohedral ])arting isdistinctly developed. Three miles east of Marshall and half a mile northeast of the mouthof Ivy
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