heport on Mineral and thermal waters. 57 



base, unless we assume this salt to have existed originally in sea- 

 water, in such a proportion as would have been seemingly in- 

 compatible with marine life. 



Mr. Lyell has also justly remarked, that the same volcanic 

 agency, which has raised the bed of the ocean, sufficiently to admit 

 of its serving as a baset* or the coral reefs which form within 

 it, also, by the carbonic acid which it causes to be emitted, oc- 

 casions a larger quantity of that calcareous matter, which they 

 require, to be dissolved by the water in their vicinity. Gypseous 

 deposits are likewise often produced by springs of the present 

 day, as noticed, with respect to those of Baden near Vienna by 

 Prevost, and that near the lake Amsanctus by myself. 



How far the beds of sulphur which occur in volcanic districts, 

 and the sulphate of lime which is associated with most beds of salt, 

 can be referred to the same, will be discussed afterwards; but we 

 must take care not to confound (as some writers appear to have 

 done,) the creative effects of mineral waters, with their decompo- 

 sing agency. The latter is illustrated in the deposits of the mud- Argiiia- 

 volcanos, as they are called, of South America, where vast masses cejus. 

 of matter, chiefly argillaceous, derived from felspathic rocks de- 

 composed by water and acid vapours, are washed down into the 

 low country, and there constitute extensive beds. 



The rocks described by Menge *, as formed by hot springs in 

 Iceland, are probably of the same description, for it is impossible 

 to follow this author in that portion of his statement, in which 

 he represents basalt, lava, and trap porphyry, as in the act of 

 being produced in them. He appeals indeed to the fact of his 

 extracting from the midst of a boiling marsh, a mass of matter, 

 which when broken, exhibited the characters of basaltic lava in 

 the centre, and towards the surface passed gradually into red and 

 grey mud ; but it seems just as easy to explain this, by the de- 

 composing influence of the water extending gradually from the 

 circumference to the centre, as by the contrary process taking 

 place in the reverse direction. 



The siliceous formations actually deposited at the present Siliceous. 

 time by springs, appear to be comparatively insignificant, the 

 most important being those of Iceland, and of St. Michael in the 

 Azores. It is probable, however, that under the sea, where the 

 influence of heat, and the chemical afiinity of alkali, are height- 

 ened by the efl"ectof an enormous pressure, beds of considerable 

 extent may be produced in this manner. 



Ii-on pyrites has been observed in a deposit from the thermal Fenugi- 

 springs of Chaudes Aigues in the Cantal, owing probably to the """^• 



* Edinh. Phi/. Journal, vol. ii. 



