GEX, a town of eastern France, chief town of an arrondissement in the department of Ain, 10 m. N.W. of Geneva and 3 m. from the Swiss frontier. Pop. (1906) town, 1385; commune, 2727. The town is beautifully situated 2000 ft. above sea-level at the base of the most easterly and highest chain of the Jura. It is the seat of a subprefect and has a tribunal of first instance, and carries on considerable trade in wine, cheese and other provisions, chiefly with Geneva. It gives its name to the old Pays de Gex, situated between the Alps and the Jura, which was at various times under the protection of the Swiss, the Genevese and the counts of Savoy, until in 1601 it came into the possession of France, retaining, however, until the Revolution its old independent jurisdiction, with Gex as its chief town. The Pays de Gex is isolated by the Jura from the rest of French territory, and comes within the circumscription of the Swiss customs, certain restrictions being imposed on its products by the French customs. GEYSER, GEISER, or GEISIR, a natural spring or fountain which discharges into the air, at more or less regular intervals of time, a column of heated water and steam; it may consequently be regarded as an intermittent hot spring. The word is the Icelandic _geysir_, gusher or rager, from the verb _geysa_, a derivative of _gjosa_, to gush. In native usage it is the proper name of the Great Geyser, and not an appellative--the general term _hver_, a hot spring, making the nearest approach to the European sense of the word (see Cleasby and Vigfusson, _Icelandic English Dictionary_, s.v.). Any hot spring capable of depositing siliceous material by the evaporation of its water may in course of time transform itself into a geyser, a tube being gradually built up as the level of the basin is raised, much in the same manner as a volcanic cone is produced. Every geyser continuing to deposit siliceous material is preparing its own destruction; for as soon as the tube becomes deep enough to contain a column of water sufficiently heavy to prevent the lower strata attaining their boiling points, the whole mechanism is deranged. The deposition of the sinter is due in part to the cooling and evaporation of the siliceous waters, and in part to the presence of living algae. In geyser districts it is easy to find thermal springs busy with the construction of the tube; warm pools, or _laugs_, as the Icelanders call them, on the top of siliceous mounds, with the mouth of the shaft still open in the middle; and dry basins from which the water has receded with their shafts now choked with rubbish. Geysers exist at the present time in many volcanic regions, as in the Malay Archipelago, Japan and South America; but the three localities where they attain their highest development are Iceland, New Zealand and the Yellowstone Park, U.S.A. The very name by which we call them indicates the historical priority of the Iceland group.