. The fluorspar deposits of southern Illinois . ) of very impure limestone containing notable quan-tities of silica and iron oxide. This banding represents horizontalbedding, and the fluorite has evidently been introduced along thestratification planes, uniting with the calcium of the limestone andexcluding the impurities. These were partly, no doubt, carried off,but also in part remain traj^ped between the growing crystals offluorite and in the bands of unreplaced limestone. These relations are perhaps made clearer by the photomicrographsof PL VI. In .1 is shown, in natural light, a portion o
. The fluorspar deposits of southern Illinois . ) of very impure limestone containing notable quan-tities of silica and iron oxide. This banding represents horizontalbedding, and the fluorite has evidently been introduced along thestratification planes, uniting with the calcium of the limestone andexcluding the impurities. These were partly, no doubt, carried off,but also in part remain traj^ped between the growing crystals offluorite and in the bands of unreplaced limestone. These relations are perhaps made clearer by the photomicrographsof PL VI. In .1 is shown, in natural light, a portion of one of thefluorspar bands. Traces of bedding may be observed, and the ironoxides and other impurities are so arranged as to mark the outlines ofgranular material, presumably the original calcite. In B the same section is shown between crossed nicols. The fluor-ite appears entirely black, while the small points of light indicate U. S. GEOLOGICAL SURVEY BULLETIN NO. 255 PL. V. INTERBANDED FLUORSPAR AND LIMESTONE FROM LEAD HILL. PLATE VI. 55 PLATE VI. PHOTO^riCROGRAPHS OF FLIT)KSrAR BANDS IN ORE BODY AT LEAD HILL. A.—Natural light, showing granular outline of original calcite particles. P,.—Polarized light, showing complete penetration of original calcite (white) by lludiiiic n;)w forming tlnorsjjar (black).C.—Natural light, showing preservation of original bedding plane in clear Uuorspar. 56 i U. S. GEOLOGICAL SURVEY BULLETIN NO. 265 PL. VI
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