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EQUATED BROWN-SPAR. *•' 109 Chemical Characters. It.hardens, and becomes dark brownish-black before the blowpipe ; and effervesces more or less briskly with acids, according to the quantity of manganese it con tains. Constituent Farts. Lime, 43.0 Magnesia, 10.0 Oxide of Iron, 8.0 Manganese, , 3.0 Water, and Carbonic Acid, . 26.5 Fcrthier, Jour, des Mines, N° 103. p. 73. Geognostic Situation. It occurs principally in veins, when it forms either the 1'redominating vein-stone, or is disseminated in the others. Ihe most frequent accompanying vein-stone is calca- reous-spar; besides which, it is often associated with heavy-spar, fluor-spar, quartz, sparry-ironstone, galena, iron-pyriles, native silver, and various ores of silver. ^ cry often it rests on all the minerals of which the vein composed : hence it is said to be the newest mineral ln the vein ; and we frequently observe thin crusts of investing the surface of crystals, as of calcareous-spar, fluor-spar, heavy-spar, quartz, galena, &c. These crusts SG hlom invest the whole crystal, generally covering only a pnrt of it; and it is observed, that it is the same side in all the crystals of the same cavity which are encrusted with the brown-spar; and also, that when the whole side “ not covered, the crust has the same height, or is on the s *>ne level in all the crystals. Geographic