TRUE VOLCANOES. 345 



coloured geognostic map of Mexico, also mentions "a subter- 

 ranean quarry of pumice-stone at Bauten," near Huichapa, 

 32 miles to the south-east of Queretaro, at a distance from 

 all volcanoes 36 . The geological explorer of the Caucasus, 

 Abich, is inclined to believe from his own observations, 

 that the vast eruption of pumice-stone near the village 

 Tschegem, in the little Kabarda, on the northern declivity of 

 the central chain of the Elburuz, is, as an effect of fissure, 

 much older than the elevation of the very distant conical 

 mountain just menioned. 



If, therefore, the volcanic activity of the earth, by radia- 

 tion of heat into space during the diminution of its original 

 temperature, and in the contraction of the superior cooling 

 strata, produces fissures and wrinkles (fractures et rides), 

 and therefore simultaneous sinking of the upper and up- 

 heaval of the lower parts 37 , we must naturally regard, as 

 the measure and evidence of this activity in the various 

 regions of the earth, the number of recognizable volcanic, 

 platforms (open, conical, and dome-shaped mountains) up- 

 heaved upon fissures. This enumeration has been repeat- 

 edly and often very imperfectly attempted : eruptive hills 



36 Federico de Gerolt, Cartas Geognosticas de los Principaks Distritos 

 Mineralcs de Mexico, 1827, p. 5. 



37 On the solidification and formation of the crusts of the earth, see 

 Cosmos, vol. i, pp. 164 166. The experiments of Bischof, Charles 

 Deville, and Delesse have thrown a new light upon the folding of the 

 body of the earth. See also the older, ingenious considerations of 

 Babbage, on the occasion of his thermit; explanation of the problem 

 presented by the temple of Serapis to the north of Puzzuoli, in the 

 Quarterly Journal of the Geological Society of London, voL iii, 1847, 

 p. 186 ; Charles Deville, Sur la Diminution de Densite dans les Roches 

 en passant de I'etat cristallin a I'etat vitreux, in the Comptes rendus 

 de VAcad. des Sciences, t. xx, 1845, p. 1453; Delesse, Sur les Effets de la 

 fusion, t. xxv, 1847, p. 455; Louis Frapolli Sur la Caractere Geologique, 

 in the Bull de la Soc. Geol. de France, 2me se>ie, t. iv, 1847, p. 627; and 

 above all, Elie de Beaumont, in his important work, Notice sur les 

 Systemes de Montagnes, 1852, t. iii. The following three sections 

 deserve the particular attention of geologists : Considerations sur les 

 Soulevements diis a une diminution lente et progressive du volume de la 

 Terre, p. 1330 ; Sur 1'Ecrasement Transversal nomme refoulement par 

 Saussure, comme une des causes dc V elevation des Chalnes de Montagnes, 

 pp. 1317, 1333, and 1346; Sur la Contraction que les Roches fondues 

 cprouvent en cristallisant, tendant des le commencement du rejroidisse- 

 ment du Globe a rendre sa masse interne plus petite que la capadte dt 

 ton enveloppe exterieure, p. 1235. 



