There are, in many parts of the world, springs, whose waters issue from their reservoirs at temperatures either at or near the boiling point of water. These are called _hot_ or _thermal springs_. Hot springs are found both in volcanic regions, as well as in regions where there are no volcanoes, but where there are lines of deep fissures or faults. According to Dana, in both of these classes, the cause is to be traced to heat of volcanic or deep subterranean origin. Hot springs are also found in regions where there are no volcanoes. In these cases the heat is due to the gradual oxidation of various sulphide ores, or to some other chemical action. The waters of hot or thermal springs almost always contain various mineral substances in solution. All spring water contains some little dissolved mineral matter, but in hot springs the quantity of this matter is greater than in cold springs, because hot water can dissolve mineral substances much better than can cold water. It might surprise you to hear that one of the commonest substances that is found in solution in the waters of many hot springs is silica; for silica is practically sand, and sand does not easily dissolve in water as does sugar. The very hot water, however, which comes from the hot spring, whose temperature below the earth's surface is very much higher than it is when it comes out of the spring, possesses the power of readily dissolving silica from the rocks over which it flows. When the waters of such springs reach the surface the silica is deposited in a solid condition around the outlets of the springs. In this way there are built up craters or mounds, or, more correctly, crater-shaped basins. Sometimes the hot water contains calcareous substances dissolved in it, the solution being caused not only by reason of the hot water, but also by means of the carbonic gas it contains. When this water flows from the springs, it builds up the same crater-shaped mounds, only in this case the mounds are of lime instead of silica. There are peculiar kinds of hot springs called _geysers_, that possess the power of throwing huge streams of water up into the air at more or less regular intervals. The word geyser is an Icelandic word meaning to rage, or snort, or gush, the name being given by reason of the manner in which the waters rush violently out during an eruption. As Dana points out, when the water in a basin of a hot spring merely boils, whether this boiling is nearly continuous, or the water is alternately boiling and quiet, the spring is called a hot or thermal spring, but where the water is thrown violently out at more or less regular intervals, it is called a geyser.