THE WORK OF GROUND-WATER. 235 



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 Tsee p. 202). Springs are also classified as min- 

 eral and common. Mineral springs, in the popular sense of the term, 

 are of two types: (1) Those which contain an unusual amount of min- 

 eral matter, and (2) those which contain some unusual mineral. Springs 

 are especially Hkely 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 con- 

 tain. Thus saline springs contain salt; sulphur springs contain com- 

 pounds (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 dis- 

 solved from limestone, or derived by chemical change from rocks con- 

 taining 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 conmion in many sedi- 

 mentary 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 qiiickly altered to iron oxide, which, 

 being relatively insoluble, is precipitated. 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 mat- 

 ter in solution varies widely. Some of the hot springs of the Yellow- 



