368 On Mineral Metamorphism. 



Metamorphosis of Focks by Vapours, 



The experiments of Jeffreys shew that watery vapours pro- 

 duce no perceptible effect upon siliceous combinations, until 

 the heat exceeds the melting point of cast-iron, but then quickly 

 decompose felspathic rocks and other silicates, and line the 

 roof of the furnace with a covering of siliceous earth resem- 

 bling hoai'-frost. The presence of siliceous earth in hot- 

 springs is explained by this in a simple way.* The long-con- 

 tinued influence of heat at low temperatures is perhaps able, 

 as frequently occurs in chemical reactions, to produce the 

 ame result as higher temperatures. The decomposition 

 which the jasper and hornstone, at M. Rotondo, adjacent 

 to the Suffioni in Tuscany, suffer from its watery vapours, 

 is in the highest degree remarkable ; the red and dark-grey 

 colours pass through different stripes into white ; the com- 

 pact texture is loosened, the stone becomes porous and pu- 

 mice-like, and crumbles at last into a mealy powder. In- 

 odorous aqueous vapour exercises the same decomposing 

 influence'on the trachyte of Terceira : the stone is changed 

 into white, fine, earthy clay, whilst the iron carried off by 

 the vapours accumulates in other places, and imparts a 

 bright-red colour to the earthy mass ; siliceous earth also 

 is extracted, and again deposited as hyalite (Darwin). " The 

 Telega-Leri in Java," says Junghuhn, " is a morass com- 

 pletely perforated by vapours ; all the rocks are decomposed 

 and metamorphosed into light-grey clay, only a few rocks 

 shew still some cohesion, but these are also bleached. 

 The water in the lake is milk-white, and in the middle is 

 about thirty feet deep, and cold. On the banks we see hun- 

 dreds of springs, of which the temperature in some is 57° C, 

 and that in others 68" C. We cannot proceed a single 

 step without coming upon hissing vapours or hot-jets, and 

 we are continually enveloped in clouds of vapour, which, 

 however, do not impede the respiration, and have only a very 

 slight smell of sulphur." Not unfrequently the decomposing 

 power of watery vapour is aided by an admixture of sulphu- 

 rous or sulphuric acid. In the neighboui'hood of the Stufe 

 di S. Calogero in Lipari, according to Hoffmann, a fume- 



* Studer's Geologie, vol. i., 247. 



