SPRINGS, natural outlets by which ground water reaches the surface. The more common small springs usually result from the outcrop of a nonporous layer below a porous stratum, the water which soaks down through the porous layer being forced to move laterally along the top of the impervious bed until it emerges. This type is usually called the hillside spring. On many steep hillsides or mountain slopes the soil acts as the porous layer, the bed rock as the Impervious stratum, and whenever the soil is thin, the water is forced to the surface forming springs or swampy places. Sometimes the water seeps slowly down into the earth along an inclined porous bed, until it reaches a crack or joint, along which it rises rapidly. Such fissure springs are often of large size, and fre quently have a high temperature, due to the great depth from which the water ascends. A third type of spring, common in limestone regions, results from the emergence of an actual underground river, which flows through a cave dissolved in the limestone. Such springs are frequently of enormous size, sometimes fur nishing power to run local mills. Warm or hot
springs are commonly termed thermal springs. The temperature varies greatly, ranging from normal to the boiling point. The heat may be due to the proximity of still uncooled volcanic rocks or to the great depth from which the water ascends along fissures. The hot springs of Yellowstone are probably of the former type, many warm springs of eastern United States appear to be of the latter sort. Practi cally all ground water carries mineral matter in solution. If this mineral matter is unusual in character or quantity, the spring is termed a mineral spring. Large quantities of lime cause calcareous springs; iron, chalybeate springs; silica, silicious springs; salt, saline or salt springs; hydrogen sulphide, sulphur springs. Many mineral springs are believed to have medicinal value, and much mineral water is bottled and sold. In 1913, mineral water to the value of $5,631,391 was reported in the United States alone.