General guide to the exhibition halls of the American Museum of Natural History . C30. HYO QUARTZ GEODES FROM URUGUAY. A geode is a nodule of stonehaving a cavitv lined with crystals of mineral matter. The rock ca\ ity in the upperfigure was encrusted in successive lavers of agate deposited from the silica dissolvedin the water which, circuated through it. Crystalline quartz was ultimately added ina final layer. This quartz, colored with a purplish hue. is known as amethyst C 33

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Keywords: ., bookcentury1900, bookdecade191, booksubjectnaturalhistorymuseums