REACTION OF THE INTERIOR OP THE EARTH 



neath the oolite, since they traverse the bunter or variegated 

 sandstone. ( 238 ) We must not confound the pre sent active 

 craters with earlier outpourings of granite, of quartzose por- 

 phyry, and of euphotide, from open temporary fissures in the 

 old transition rocks. 



The cessation of volcanic activity may be only partial, 

 the subterranean fire finding for itself another outlet in the 

 same chain of mountains ; or it may be total, as in Au- 

 vergne. More recent examples of the latter class are known 

 to have occurred within historic periods; the volcano of 

 Mosychlos, () on the island consecrated to Vulcan, of 

 which the " high whirling flames" were still known to So- 

 phocles ; and the volcano of Medina, which, according to 

 Burckhardt, sent forth a stream of lava as late as the 2d of 

 November, 1276. * Each stage of volcanic activity, from its 

 first excitement to its extinction, is characterised by a parti- 

 cular class of products : first, by ignited scoriae, by currents 

 of lava consisting of trachyte, pyroxene, and obsidian, and 

 by rapilli and tufaceous ashes, accompanied by an abun- 

 dant disengagement of steam, usually quite pure : at a later 

 period the volcano becomes a solfatara, where aqueous 

 vapours are emitted mixed with sulphuretted hydrogen and 

 carbonic acid gases : and lastly, the crater becomes entirely 

 cooled, and carbonic acid exhalations only proceed from it.- 

 There is an extraordinary class of volcanoes (such as the 

 Galungung, in the island of Java), which do not emit lavas, 

 but only devastating streams of boiling water, accompanied 

 by sulphur in combustion, and rocks reduced to the state of 

 dust ; ( 24 ) but whether these present a normal condition, or 

 are only a certain transitory modification of volcanic processes, 

 must remain undecided until they are visited by geologists 

 skilled in the doctrines of modern chemistry. 



