Springs are classified in various ways, and these several classifications suggest characteristics worthy of note. They are sometimes said to be _deep_ and _shallow_. The “deep” spring, as the term is ordinarily used, is one which issues with great force, and with something of upward movement, and the “shallow” spring, one which issues with little force, and without upward movement; but the spring which issues with force is not necessarily deep, nor is the one which issues with little force necessarily shallow. The idea involved in this grouping would be better expressed by _strong_ and _feeble_. Springs are also classified as _cold_ and _thermal_, the latter term meaning simply that the temperature is such as to make the springs seem warm or hot. The temperature of thermal springs ranges up to the boiling-point of water. Between deep springs and shallow ones, and between cold springs and thermal, respectively, there is no sharp line of demarkation. Again, some springs are continuous in their flow, while others are _intermittent_. Most intermittent springs flow after periods of precipitation, but dry up during droughts (see p. 202). Springs are also classified as _mineral_ and _common_. Mineral springs, in the popular sense of the term, are of two types: (1) Those which contain an unusual amount of mineral matter, and (2) those which contain some unusual mineral. Springs are especially likely to be called mineral if the substances which they contain, have, or are supposed to have, some medicinal property. All springs which are not “mineral” are “common.” This classification is not altogether rational, for all springs contain more or less mineral matter, and many springs which are “common,” contain more mineral matter than some springs that are “mineral.” Mineral springs are themselves classified according to the kind and amount of mineral matter they contain. Thus _saline_ springs contain salt; _sulphur_ springs contain compounds (especially gaseous) of sulphur; _chalybeate_ springs contain iron compounds, especially the sulphate; _calcareous_ springs contain abundant lime carbonate, etc. These various mineral substances are extracted from the rock, sometimes by simple solution, and sometimes by solution resulting from other chemical change. The salt of saline springs is usually extracted from beds of salt beneath the surface. Lime carbonate, one of the commonest substances in solution in ground-water, is dissolved from limestone, or derived by chemical change from rocks containing other calcium compounds. Thus lime feldspars, by carbonation, give rise to lime carbonate. The chalybeate waters often arise from the oxidation of iron sulphide, a mineral which is common in many sedimentary rocks. The iron sulphate is itself subject to change in the presence of the ubiquitous lime carbonate. From this change iron carbonate results, and this is usually quickly altered to iron oxide, which, being relatively insoluble, is precipitated.