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A kind of glass produced by volcanoes. It is usually of a black color, and opaque, except in thin splinters.

rystalline body having a radiated structure, observed in some vitreous volcanic rocks, as obsidian and pearlstone.

A kind of crystallite resembling a bunch of hairs, common in obsidian. See Illust. of Crystallite.

e most active. -- Volcanic glass, the vitreous form of lava, produced by sudden cooling; obsidian. See Obsidian. -- Volcanic mud, fetid, sulphurous mud discharged by a volcano. -- Volcanic rocks, rocks which have been produced from the discharges of volcanic matter, as the various kinds of basalt, trachyte, scoria,…