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350 ~ DYNAMICAL GEOLOGY. [Boor TIT. fa 

exposure and evaporation, the carbonate is thrown down as a white 
precipitate. Water saturated with carbonic acid will at the freezing 
point dissolve 0°70 gramme and at 10° C., 0°88 gramme of calcium 
carbonate per litre. Calcareous springs occur abundantly in lime- 
stone districts, and indeed may be looked for wherever the rocks 
are of a markedly calcareous character. In some regions they — 
have brought up such enormous quantities of lime as to form con- 
siderable hills (postea, p 354). | 
Ferruginous or Chalybeate Springs contain a large proportion of 
iron in the total mineral ingredients, and are known by their inky 
taste, and the yellow, brown, or red ochry deposit along their 
channel. They may be frequently observed in districts where beds 
or veins of ironstone occur, or-where the rocks contain much iron in 
combination, particularly in the waters of old mines. In many cases 
the iron is supplied by the weathering of the sulphide (marcasite) 
so abundantly contained among stratified rocks. Ferrous sulphate 
is produced and brought to the surface, but in presence of carbonates, 
particularly of the ubiquitous carbonate of lime, this sulphate is 
decomposed, the acid being taken up by the alkaline earth or alkali 
and the iron becoming a ferrous carbonate, which rapidly oxidizes 
and falls as the familiar yellow or brown crust of hydrous peroxide. 
The rapidity with which ferrous-carbonate is thus oxidized and 
precipitated was well shown by Fresenius in the case of the Langen- 
schwalbach chalybeate spring. In its fresh state the water contains _ 
in 1000 parts 0°37696 of protoxide of iron. After standing twenty- 
four hours it was found to contain only 87:7 per cent. of the original 
amount of iron; after sixty hours 62°9 per cent., and after eighty-four 
hours 53:2 per cent.’ 
Brine Springs (Soolquellen) bring to the surface a solution in 
which sodium chloride greatly predominates, Springs of this kind 
appear where beds of solid rock-salt exist underneath, or where the 
rocks are impregnated with the mineral. Most of the brines worked 
as sources of salt are derived from artificial borings into saliferous 
rocks. Those of Cheshire in England, the Salzkammergut in 
Austria, Bex in Switzerland, &., have long been well known. Some 
of the English brines contain about one per cent. of salts, of which 
chloride of sodium may range from a half to three-fourths or more. 
Other brines, however, yield a far larger amount; one at Clemens- 
hall, Wiirtemberg, gave upwards of 26 per cent. of salts, of which 
almost the whole was chloride of sodium, ‘The other substances 
contained in solution in the water of brine springs are chlorides of 
potassium, magnesium, and calcium; sulphates of calcium and less — 
frequently of sodium, potassium, magnesium, barium, strontium, or 
aluminium; silica; compounds of iodine and fluorine; with phos- 
phates, arseniates, borates, nitrates, organic matter, carbon dioxide, 
sulphuretted hydrogen, marsh gas, and nitrogen.” 
' Journal fiir Prakt. Chem. \xiv. 368, quoted by Roth, op. cit, i. p. 565. 
* Roth, Chem. Geol. i. p. 442, Bischot, Chem, Geol. ii, Ray 
