396 COSMOS. 



been occupied, having in all cases consulted the original 



General Victor Hugues par Amic et Hapel sur le Volcan de la Basse 

 Terre, dans la ntiit du 7 au 8 Vendemiaire, an G, pag. 46 ; Humboldt, 

 Voyage, t. i., p. 316). The lower part of the mountain is dioritic rock; 

 the volcanic cone, the summit of which is open, is trachyte, containing 

 labradorite. Lava does not appear even to have flowed in streams 

 from the mountain called, on account of its usual condition, the Sou- 

 friere, either from the summit crater or from the lateral fissures, but 

 the ashes of the eruptions of Sept., 1797, Dec., 1836, and Feb., 1837, 

 examined by the excellent and much lamented Dufrenoy, with his pe- 

 culiar accuracy, were found to be finely pulverized fragments of lava, 

 in which feldspathic minerals (labradorite, rhyakolite, and sanidine) 

 were recognizable, together with pyroxene. (See Lherminier, Daver, 

 Elie de Beaumont, and Dufrenoy, in the Comptes rendus de I'Acad. des 

 Sc., t. iv., 1837, p. 294 ; 651 and 743-749). Small fragments of quartz 

 have also been recognized by Deville in the trachytes of the soufriere, 

 together with the crystals of labradorite (Comptes rendus, t. xxxii., p. 

 675), while Gustav Rose even found hexagonal dodecahedra of quartz 

 in the trachytes of the volcano of Arequipa (Meyen, Reise um die Erde, 

 bd. ii., s. 23). 



The phenomena here described, of the temporary ejection of very 

 various mineral productions from the fissure openings of a soufriere, 

 remind us very forcibly that what we are accustomed to denominate a 

 solfatara, soufriere, or fumarole denotes, properly speaking, only cer- 

 tain conditions of volcanic action. Volcanoes which have once emit- 

 ted lava, or, when that failed, have ejected loose scoriae of considera- 

 ble volume ; or, finally, the same scoria; pulverized by trituration, pass, 

 on a diminution of their activity, into a state in which they yield 

 only sulphur, sublimates of sulphurous acid, and aqueous vapor. If 

 as such we were to call them semi-volcanoes, it would readily convey 

 the idea that they are a peculiar class of volcanoes. Bunsen, to whom, 

 along with Boussingault, Senarmont, Charles Deville, and Danbree, 

 science is indebted for such important advances for their ingenious 

 and happy application of chemistry to geology, and especially to the 

 volcanic processes, shows " how, when in sulphur sublimations, whjch 

 almost always accompany volcanic eruptions, the masses of sulphur in 

 the form of vapor come in contact with the glowing pyroxene rocks, 

 the sulphurous acid is generated by the partial decomposition of the 

 oxyd of iron contained in those rocks. If the volcanic action then 

 sinks to a lower temperature, the chemical action of that zone then 

 enters into a new phase. The sulphurous combinations of iron, and 

 perhaps of metals of the earths and alkalies there produced, com- 

 mence their operation on the aqueous vapor, and the result of the al- 

 ternate action is the generation of sulphureted hydrogen and the prod- 

 ucts of its decomposition, disengaged hydrogen and sulphur vapor." 

 The sulphur fumaroles outlive the great volcanic eruptions for centu- 

 ries. The muriatic acid fumaroles belong to a different and later pe- 

 riod. They seldom assume the character of permanent phenomena. 

 The muriatic acid in the gases of craters is generated in this way : the 

 common salt which so often occurs as a product of sublimation in vol- 

 ca^noes, particularly in Vesuvius, is decomposed in higher tempera- 

 tures, under the co-operation of aqueous vapor and silicates, and forms 

 muriatic acid and soda, the latter combining with the silicates present. 



