Ch. XXXV.] ROCKS ALTERED BY SUBTERRANEAN GASES. 741 



under the influence of heat by water holding alkaline silicates in solution, 

 as in the case of the Plornbieres springs, without any chemical reaction. 

 The quantity of water required, according to Daubree, to produce great 

 transformations in the mineral structure of rocks, is very small. As to 

 the heat required, silicates may be produced in the moist way at about 

 incipient red heat, whereas to form the same in the dry way would 

 require a much higher temperature. 



M. Fournet, in his description of the metalliferous gneiss near Cler- 

 mont, in Auvergne, states that all the minute fissures of the rock are 

 quite saturated with free carbonic acid gas ; which gas rises plentifully 

 from the soil there and in many parts of the surrounding country. 

 The various elements of the gneiss, with the exception of the quartz, 

 are all softened ; and new combinations of the acid with lime, iron, 

 and manganese are continually in progress.* 



Another illustration of the power of subterranean gases is afforded 

 by the Stufas of St. Calogero, situated in the largest of the Lipari 

 Islands. Here, according to the description published by Hoffmann, 

 horizontal strata of tuff, extending for four miles along the coast, and 

 forming; cliffs more than 200 feet kio\h have been discolored in various 

 places, and strangely altered by the " all-penetrating vapors." Dark 

 clays have become yellow, or often snow-white; or have 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 chalcedony and opal, and others of fibrous gypsum, have 

 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 decomposition 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 

 rock, which intervene between the subterranean reservoirs of gas and 

 the external air. The extent, therefore, of the earth's crust which the 

 vapors have permeated and are now permeating may be thousands of 

 fathoms in thickness, and their heating and modifying influence may 

 be spread throughout 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 167° F., 

 has converted the surface of some blocks of black marble into a 



* See "Principles," Index, "Carbonated Springs," &c. 



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



% See Princ. of Geol. ; and Bulletin de la Soc. Geol. de France, torn. ii. p. 230. 



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



