Geode, a hollow shell of stone, commonly of quartz, found in various rocks, and usually lined with crystals pointing toward the centre. These crystals are for the most part of quartz, often amethystine. Among the amygdaloids of the trap are found geodes of agate and chalcedony, the shell made up of concentric layers of these variously colored silicious matters. Besides quartz crystals, others of calcareous spar, analcime, etc, are found in the cavities of geodes. Some of the most remarkable specimens of this kind in the quartz geodes are found in low stages of water loose in the rapids of the upper Mississippi river. Externally they are rough and unsightly, of light brown color, and of all sizes up to 12 or 15 inches in diameter; when broken they present beautiful groups of quartz crystals. Water is sometimes found in the geodes holding the silex in solution, and making with it a milky-looking mixture. As the water evaporates the silex has been known to suddenly form into delicate crystals. Such geodes were at one time abundantly found on Brier creek in Scriven or Burke co., Ga., in a rock composed of hornstone and jasper; the milky fluid contained in them was used by the inhabitants as a paint or whitewash. (" American Journal of Science," vol. viii., p. 286.)