Bulletin of the United States Geological Survey--Corundum and Its Occurrence and Distribution in the United States (A Revised and Enlarged Edition of Bulletin ) . he peridotite formation close to the (contactof that rock with the gneiss. These dikes vary in width from 25 toover 300 feet, their average width being from 75 to 100 feet. Therelation of these amphibolite dikes to the peridotite formation atBuck Creek, Clay County, N. C, is shown in fig. 9. «Mon. U. S. Geol. Survey, vol. 29, 1898, pp. 47-54. 36 CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. The gTOundmass of this amphibolite is a
Bulletin of the United States Geological Survey--Corundum and Its Occurrence and Distribution in the United States (A Revised and Enlarged Edition of Bulletin ) . he peridotite formation close to the (contactof that rock with the gneiss. These dikes vary in width from 25 toover 300 feet, their average width being from 75 to 100 feet. Therelation of these amphibolite dikes to the peridotite formation atBuck Creek, Clay County, N. C, is shown in fig. 9. «Mon. U. S. Geol. Survey, vol. 29, 1898, pp. 47-54. 36 CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. The gTOundmass of this amphibolite is a grass-green ainphibole,containing IT per cent of akuiiina, nearly 12 per cent of lime, andone-half of 1 per cent of magnesia, which is best classified under theedenite variety of aluminous amphiboles. The rich green color of theedenite is undoubtedly due to the presence of a small amount of chro-mic oxide, the analysis showing the presence of per cent of thisoxide. Many microscopic grains of picotite or chromite are scatteredthrough the groundmass of edenite. There is also present, in widelyvarying proportions, the plagioclase feldspar anorthite. The feld-. CONTOUrf INTCRVAL 50 FEET Fig. 9.—Map of the Buck Creek peridotite 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 amphibolit
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