70 BUNSEN ON THE FORMATION OF 



insignificant in amount, and its former existence is indicated by- 

 all the signs of a previous state of ignition. The nature of this 

 action is in part essentially determined by the substance of the 

 rock heated. The readily fusible palagonitic tuff is converted 

 into basalt and zeolitic conglomerate, limestone to a superbasic 

 silicate — the material of the palagonitic formations — sandstone 

 to a vitrified mass resembling hornstone, in which the eruptive 

 rock frequently pours through capillary rents and fissures in the 

 form of an extravasated interfused mass. 



Besides these two modes of action, there is a third, that, namely, 

 in which the boiling-point of the water is raised by enormous 

 pressure, even to a red heat, in consequence of which direct 

 interfusion products of red-hot melted rock with red-hot liquid 

 water are formed. There are, indeed, phaenomena in Iceland 

 w^hich appear scarcely to admit of any other explanation than 

 this. I shall return to the consideration of this point at some 

 future opportunity. 



These remarks will suffice to show that the greater number 

 of processes of plutonic interfusion and metamorphosis must be 

 preceded by a vaporization of water. The mechanical effects 

 of this change present themselves in the volcanic concussions 

 and eruptions, the chemical effects in the manifold exercise of 

 fumarole action. Consequently the study of this action, and the 

 products resulting from it, is of especial interest for the theory of 

 volcanoes. The volcanic actions continuing after eruptions and 

 manifesting themselves in solfataras, geysers, and thermal springs, 

 furnish the material, which renders it possible, by means of direct 

 observation and experiment, to penetrate to the source of all 

 these phaenomena connected in the most intimate manner with 

 internal volcanic activity. 



The most important data for such an investigation consist in 

 a knowledge of the composition of the exhalations, which, as con- 

 sequences of the great volcanic catastrophes, issue from the earth 

 in fumaroles. 



Besides aqueous vapour, which constitutes the chief part of 

 these exhalations, they contain as gaseous constituents only car- 

 bonic acid, hydrochloric acid, sulphur vapour, sulphuretted hy- 

 drogen, sulphurous acid, free hydrogen, and, together with these 

 as foreign admixtures not properly volcanic, nitrogen, oxygen 



