About chalybeate springs, therefore, iron oxide is frequently being deposited. _Medicinal_ springs are those which contain some substance or substances which have, or are supposed to have, curative properties. =Mineral matter in solution.=—The number and variety of mineral substances in spring water is very great, and the amount of solid matter in solution varies widely. Some of the hot springs of the Yellowstone Park contain nearly three grams (2.8733) of mineral matter per kilogram.[108] The composition of various spring and well waters is shown in the accompanying table, which gives some idea of the range of mineral substances commonly in solution in ground-water. =Geysers.=—Geysers are intermittently eruptive hot springs. They occur only in volcanic regions (past or present) and in but few of them. Active geysers are virtually confined to the Yellowstone Park and Iceland, though they formerly existed at other places. Those of New Zealand have but recently become extinct. The great geyser region of the world is the Yellowstone Park, where there are said to be more than sixty active geysers. The cause of the eruption is steam. The surface-water sinks down until, at some unknown depth, it comes into contact with rock sufficiently hot to boil it. The source of the heat is not open to inspection, but it is believed to be the uncooled part of an extrusive lava flow, or of an intrusive lava mass. From what was said on pp. 216 and 217 it is clear that geysers do not have their origin in water which sinks down to the zone of great heat, where the increment of heat is normal. The water of a geyser issues through a tube of unknown length. Whether the tube is open down to the source of the heat is not determinable, but water from such a source finds its way to the tube. Water may enter the tube from all sides and at various levels from top to bottom. The heating may precede or follow its entrance into the tube, or both. So far as the water is heated after it enters the tube, the point of most rapid heating may be at the bottom of the tube or at some point above. If the temperature of the source of heat were high enough to convert the descending water into steam as fast as it enters the tube, the steam would escape continuously, though there would be no geyser; but if the rock is only hot enough to bring the water to the boiling-point after some lapse of time, and after some water has accumulated, an eruption is possible. ANALYSES OF AMERICAN SPRING-WATERS.[109] [+Reduced to Parts per 1000 by Dr H. J. Van Hoesen.+]