Springs

Springs, the risings of underground waters to the surface of the ground, may be broadly divided into two classes - surface or gravitation springs, where the water descends continuously to the point of outflow, and deep-seated springs, where it rises by hydrostatic pressure. Those of the first class mostly occur in undisturbed strata, where a porous bed cropping out at the surface receives rainfall; its water is held up by an underlying impermeable bed, and at some lower level the line of junction of the two beds comes to the surface. The second, more common, class occurs in disturbed areas, the water following a labyrinthine up-and-down course through subterranean fissures and joints, and often reaching the surface along a line of fault. The water of springs may range in temperature almost from the freezing-point, as in some glacieres or deep caverns in snow-clad mountains, up to boiling-point. Hot or thermal springs are most frequent in volcanic regions, but may occur elsewhere (as at Bath, where they have a temperature of about 120° F.), when they probably come from a considerable depth. Spring water contains in solution atmospheric gases, carbon dioxide from the soil, and various gases from deeper rocks. Organic acids may be present, and mineral constituents mainly vary in proportion according to temperature, from less than 1 to 300 grams per litre. The chief mineral salts present are calcium, magnesium, and sodium carbonates, calcium and sodium sulphates, and sodium chloride. When more than 1 gram per litre is present the water ceases to be ordinary drinking water, and the spring is termed a mineral spring. Where drinking-water only contains alkaline salts and dissolves soap without forming curd it is termed soft, but where calcium, magnesium, or ferrous carbonates, sulphates, or chlorides are present, curd is formed from the fatty acids of soap, and the water is called hard. Hardness due to bicarbonates, which are decomposed by boiling, the carbonate being precipitated, is termed temporary; that due to the undecomposable sulphates and chlorides as permanent. The chief kinds of mineral springs are calcareous, containing calcium-carbonate; chalybeate or ferruginous, containing ferrous sulphate, which decomposes and deposits iron rust (hydrous oxide); or saline, containing a brine chiefly of chlorides, with calcium sulphate and various other substances. Mineral springs believed to have curative effects are called medicinal; of which the chief varieties are the sulphurous containing hydrogen-sulphide, as at Harrogate; the bitter, containing magnesium-sulphate, as at Cheltenham and Sedlitz; and the alkaline, containing especially sodium-carbonate. Oil-springs contain a variable proportion of petroleum (q.v.) mixed with their water. In addition to feeding rivers, the chief geological action of springs (at the surface) is the deposition of travertine (q.v.), which when rapid gives them the name of petrifying springs, though they merely encrust with carbonate of lime. Chalybeate springs produce an ironstone moorband pan below the surface in badly drained districts, and the hot waters of geysers contain in solution large quantities of silica, which they deposit as geyserite or siliceous sinter.