Ch. xxxv.] rocks alteeed by subterranean gases. 595 



and new combinations of the acid with hme, iron, and manganese are 

 continually in progress.* 



Another illustration of the power of subterranean gases is afforded by 

 the stufas of St. Galogero, situated in the largest of the Lipari Islands. 

 Here, according to the description published by Hoffmann, horizontal 

 strata of tuff, extending for 4 miles along the coast, and forming cliffs 

 more than 200 feet high, have been discolored in various places, and 

 strangely altered hy^ the " all-penetrating vapors." Dark clays have be- 

 come yellow, or often snow-whit^ ; or hav-e assumed a chequered or 

 brecciated appearance, being crossed with ferruginous red stripes. In 

 some places the fumeroles have been found by analysis to consist partly 

 of sublimations of oxide of iron ; but it also appears that veins of chalce- 

 dony and opal, and others of fibrous gypsum, hav€ resulted from these 

 volcanic exhalations.f 



The reader may also refer to M. Virlet's account of the corrosion of 

 hard, flinty, and jaspideous rocks near Corinth by the prolonged agency 

 of subterranean gases ;J; and to Dr. Daubeny's description of the decom- 

 position of trachytic rocks in the Solfatara, near Naples, by sulphuretted 

 hydrogen and muriatic acid gases.§ 



Although in all these instances we can only study the phenomena as 

 exhibited at the surface, it is clear that the gaseous fluids must have 

 made their way through the whole thickness of porous or fissured rocks, 

 Avhich intervene between the subterranean reservoirs of gas and the exter- 

 nal air. The extent, therefore, of the earth's crust which the vapors have 

 permeated and are now permeating may be thousands of fathoms in thick- 

 ness, and their heating and modifying influence may be spread through- 

 out the whole of this solid mass. 



We learn from Professor Bischoff that the steam of a hot spring 

 at Aix-la-Chapelle, although its temperature is only from 133° to 

 16 '7° F., has converted the surface of some blocks of black marble 

 into a doughy mass. He conceives, therefore, that steam in the bow- 

 els of the earth, having a temperature equal or even greater than 

 the melting point of lava, and having an elasticity of which even Pa- 

 pin's digester can give but a faint idea, may convert rocks into liquid 

 matter. II 



The above observations are calculated to meet some of the objections 

 which have been urged against the metamorphic theory on the ground 

 of the small power of rocks to conduct heat ; for it is well known that 

 rocks, when dry and in the air, differ remarkably from metals in this 

 respect. It has been asked how the changes which extend merely for a 

 few feet from the contact of a dike could have penetrated through moun- 



* See Principles, Index, "Carbonated Springs," tke. 

 f Hoffmann's Lipaiischen Inseln, p. 38. Leipzig, 1832. 



j See Princ. of Geol. ; and Bulletin de la Soc. Geol. de France, torn, ii 

 p. 230. 



§ See Princ. of Geol. ; and Daubeny's Volcanos, p. 167. 

 I Jam. Ed, New Phil. Journ. No. 51, p. 43. 



