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Show Origin of Onyx. When waters charged with carbo-nato carbo-nato of calcium derived from lime-Btono lime-Btono nro allowed to ovaparato they deposit their load It tho form of sinter, sin-ter, or tufn. This process can bo observed ob-served at mnny thormal and "petrifying" "petri-fying" springs and also In tho formation for-mation of stalactites and stalagmites In Umcstono caverns. In this wny large masses of compact enrbonato aro formed, somo of them of great beauty. Tho so-called "onyx marbles," mar-bles," of which tho Moxlcan "onyx" Is a familiar example, aro formed In this way. Somo rock of this class Is ctalagmntlc, In caverns, nnd somo of It Is formed by springs. Its variations va-riations In color and tcxturo, to which Its ornamental character is largely duo, aro commonly produced by Impurities Im-purities or Inclusions, f.uch us oxldo of Iron, or oven mud und clay. United States Geological Survey, |