410 



FERRUGINOUS AND BRINE SPRINGS. 



[Ch. XVII. 



in an insoluble state takes place partly because the water 

 when cooled by exposure to the air is unable to retain as 

 much silica as when it issues from the earth at a temperature 

 of 180° or 190° Fahr. ; and partly because the evaporation of 

 the water decomposes the compound of silica and soda which 

 previously existed. This last change is probably hastened 



byth 





atmosphe 



The alkali, when disunited from the silica, would readily be 

 dissolved in and removed by running water.* 



Mineral waters, even when charged with a small propor- 

 tion of silica, as those of Ischia, 



may 



matter for their sili- 



ceous secretions ; but there is little doubt that rivers obtain 

 silex in solution from another and far more general source, 



decomposition 



When 



which is so abundant an ingredient in the hypogene and trap- 

 pean rocks, has disintegrated, it is found that the residue, 

 called porcelain clay, contains a small proportion of the silica 

 which existed in the original felspar, the other part having 

 been dissolved and removed by water. f 



Ferruginous springs. — The waters of almost all springs con- 

 tain some iron in solution ; and it is a fact familiar to all, 

 that many of them are so copiously impregnated with this 

 metal, as to stain the rocks or herbage through which they 

 pass, and to bind together sand and gravel into solid masses. 

 We may naturally, then, conclude that this iron, which is 

 constantly conveyed from the interior of the earth into lakes 

 and seas, and which does not escape again from them into 

 the atmosphere by evaporation, must act as a colouring and 

 cementing principle in the subaqueous deposits now in pro- 

 gress. Geologists are aware that many ancient sandstones 

 and conglomerates are bound together or coloured by iron. 



Brine springs. — So great is the quantity of muriate of soda 

 in some springs, that they yield one fourth of their weight 

 in salt. They are rarely, however, so saturated, and generally 

 contain, intermixed with salt, carbonate and sulphate of lime* 

 magnesia, and other mineral ingredients. The brine springs 



and Dr. Turner, Jam. Ed. New P1» L 



* Barrow's Iceland, p. 209. 



f See Lyell's Elements of Geology; Journ. No. xxx. p. 216. 



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