THE REVOLUTIONS OF THE CRUST OF THE EARTH. 299- 



volcauoey of Java have given place to emanatious formed only of the 

 debris of ancient eruptive rocks mingled with cinders, and reduced to 

 a pasty condition by the water which penetrates into the interior ot the 

 crater. He establishes three periods of activity for these volcanoes, 

 characterized, first, by emissions of trachytic lava (oligoklasaugit) in 

 the incandescent but pasty condition, which extend horizontally in 

 strata and form the volcanic cone; second, by the flowing out of tra- 

 chytic rarely basaltic lava, in a fluid state ; and third, by the expulsion of 

 cinders, sand, and angular fragments of incandescent lava, which takes 

 place still in onr day. The accumulation of water in the craters causes 

 the eruptions of mud. Very remarkable are the aqueous eruptions 

 observed in South America. Bouguer reports that at the time of the 

 eruption of the volcano Cotopaxi, which occurred in 1742, a disastrous 

 inundation was attributed to the water thrown up by the volcano. An 

 analogous phenomenon was presented at Lancerotte, one of the Canary 

 Islands. There it could not be said, as in the case preceding, that the 

 water came from the melting of the snow accumulated upon the cone of 

 the volcano, for, in the first place, there was no snow upon the island, 

 and, in the second, the water was seen to come out of several, of the 

 craters in a state of activity.* 



According to M. Saint-Claire-Deville,t the phases of volcanic action 

 are three in number, first, phase of great activity, marked by violent 

 eruptions, such as Vesuvius has experienced several times within eight- 

 een centuries ; second, phase of mean activity or stromholian phase, char- 

 acterized by slight eruptions, succeeding each other at short intervals ; 

 these erupt'ions have their seat at the summit of the cone or in its im- ■ 

 mediate neighborhood ; third, phase of least activity or sulphureous phase, 

 when only aqueous vapors are disengaged, accompanied, according to 

 the intensity of the eruption, with chlorhydric, sulphurous, sulphydric 

 carbonic acids, hydrogen gas, and carbureted hydrogen. Not only do we 

 observe a certain succession of these products during the sulphureous 

 phase, but we find also that they are disengaged at a determinate dis- 

 tance from the opening and in concentric circles. Nearest the mouth 

 the chlorhydric acid, then the sulphurous acid, the sulphydric acid, the 

 carbonic acid, and lastly the carbureted hydrogen. This succession 

 seems to indicate that the compounds of sulphur and carbon, which are 

 thus disengaged, around the cone of the eruption, are only metamor- 

 phic products, which are formed by action of the heat from the lava, 

 arrested in the volcanic passages, upon the rocks impregnated with 

 water, which contain sulphur and carbon in a state of combination. 



Volcanoes must have existed at every age of the world, but of course 

 they diflered greatly in importance, when the terrestrial crust was 

 hardly formed, until now when it is about 40 kilometers (25 miles) in 

 thickness. However, M. d'Ormalius d'Halloy affirms that volcanoes did 

 not exist before the Quaternary period.^ Ami Boue, on the contrary , 



* Delia Torre, Storia itfcnomeni del Vesuvio, p. 82. iCompies Eendtis, t. Ixvi, p. 80:5. 



^Bulletin of llw Geological Societij of France, t. xi, p. 80, 1853. 



