bYNAMICAL GEOLOGY. 279 



Suggested sulphur as the cause of fluxion, Spallanzani believed 

 that the expansion of vapours was the main cause of the 

 explosive phenomena of eruptions, and Humboldt and 

 Poulett-Scrope accepted and extended Spallanzani's view. 

 Scrope regarded the elastic vapours as original constituents 

 of the earth-magma ; on the other hand, Humboldt contended 

 that water had passed down from the surface through fissures, 

 had there come in contact with the glowing magma, been 

 converted into steam, and absorbed in the magma. The 

 majority of later geologists agree with Humboldt's explana- 

 tion. 



Humboldt had chiefly in view the descent of sea-water 

 through crust-fissures, as the geographical distribution of 

 active volcanoes would suggest, but he by no means excluded 

 the likelihood that similar results ensue from the percolation 

 of meteoric water through the rocks. The obvious diffi- 

 culty, pointed out by Humboldt himself, was whether the 

 hydrostatic pressure of the descending column of water could 

 overcome the resistance of the vapours at high tension in 

 the earth's interior. Bischof and Daubree have shown that 

 surface water may, in virtue of capillarity and the pressure of 

 its own column, descend into the heated depths of the earth. 

 Angelot also concluded that the tension of a column of water 

 would at any depth be overcome by the pressure of the 

 superincumbent masses of water; in his opinion, the ocean 

 is the source of the vapours dissolved in deep-seated magma. 

 And Bischof shows that not only, water-vapour but also 

 carbonic acid, hydrochloric acid, and other gases imprisoned 

 in rock-magma play a considerable part in eruption. 



In more recent geological writings, Reyer has investigated 

 the question of supply in reference to the constituents of 

 molten magmas, and his conclusions are in agreement with 

 those of Angelot, Fourier, and Poulett-Scrope. According to 

 Reyer, at the formation of the earth, not only vapour of water, 

 but many other gases and liquids were intermixed with the 

 material matter of the earth, and these have been preserved in 

 it. The continual separation of the less fusible parts from the 

 magma is always accompanied by the escape of gases. These 

 are absorbed by the liquids with which the magmas are 

 soaked, and owing to a relief of superincumbent pressure,- the 

 liquids may at any time vaporise and the magma may be 

 expelled towards the surface in fluid condition. Experiments 



