PART I. CHAPTER XL 



147 



Rocks altered by Subterranean Gases. 



their pores. Although the gaseous matter first absorbed would 

 soon be condensed, and part with its heat, yet the continued arri- 

 val of fresh supplies from below, might, in the course of ages, 

 cause the temperature of the water, and with it that of the con- 

 taining rock, to be materially raised. 



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

 Clermont, in Auvergne, states that all the minute fissures of the 

 rock are quite saturated with free carbonic acid gas, which rises 

 plentifully from the soil there and in many parts of the surround- 

 ing country. The various elements of the gneiss, with the ex- 

 ception 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 af- 

 forded by the stufas of St. Calogero, situated in the largest of the 

 Lipari Islands. Here, according to the description lately pub- 

 lished by Hoffmann, horizontal strata of tuff, extending for four 

 miles along the coast, and forming cliffs more than 200 feet high, 

 have been discoloured in various places, and strangely altered 

 by the *' all-penetrating vapours." Dark clays have become yel- 

 low, or often snow-white ; or have assumed a chequered and 

 brecciated appearance, being crossed with ferruginous red stripes. 

 In some places the fumeroles have been found by analysis to con- 

 sist partly of sublimations of oxide of iron ; but it also appears 

 that veins of calcedony and opal, and others of fibrous gypsum, 

 have resulted from these volcanic exhalations."]" 



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

 sion of hard, flinty, and jaspideous rocks near Corinth, by the 

 prolonged agency of subterranean gases and to Dr. Daubeny's 

 description of the decomposition of trachyte rocks in the Solfa- 

 tara, near Naples, by sulphuretted hydrogen and muriatic acid 

 gases. § 



Although in all these instances we can only study the phe- 

 nomena 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, which intervene between the subterra- 

 nean reservoirs of gas and the external air. The extent, there- 

 fore, of the earth's crust, which the vapours have permeated and 

 are now permeating, may be thousands of fathoms in thickness, 



* See Principles of Geology, Index, " Auvergne," &c. 

 t Hoffmann's Liparischen Inseln, p. 38. Leipzig, 1832. 



X See Principles of Geolgy ; and Bulletin de la Soc. Geol. de France, torn. ii. 

 p. 330. 



$ See Principles of Geology ; and Daubeny's Volcanos, p. 167. 



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