VESUVIUS IN ERUPTION 71 



bacteria." Whether the neighbouring great volcano had 

 any share in the process seems to be doubtful. 



It is generally supposed that sea-water makes its way 

 in large quantity through fissures connected with volcanic 

 channels, and is one of the agents of the explosions 

 caused by the subterranean molten matter. Gaseous 

 water, hydrochloric acid, carbonic acid, hydrofluoric acid, 

 and even pure hydrogen and oxygen and argon are 

 among the gases ejected by volcanoes. 



The molten matter forced up from the bowels of the 

 earth and poured out by volcanoes is made up of various 

 chemical substances, differing in different localities, and 

 even in different eruptions of the same volcano. It 

 consists largely of silicates of iron, lime, magnesium, 

 aluminium, and the alkali metals, with possible admixture 

 of nearly every other element. Some volcanoes eject 

 " pitch " or " bitumen." When the molten matter cools, 

 interesting crystals of various "species" (i.e., of various 

 chemical composition) usually form in the deeper part of 

 the mass. The lavas of Vesuvius frequently contain 

 beautiful opaque-white twelve-sided crystals of a siliceous 

 mineral called " leucite." I have collected in the lava of 

 Niedermendig, on the Rhine, specimens embedding bright 

 blue transparent crystals (a mineral called Haliynite) 

 scattered in the grey porous rock. The lava-streams, and 

 even the " roots," of extinct volcanoes which are of great 

 geologic age, sometimes become exposed by the change of 

 the earth's surface, and extensive sheets of volcanic rock 

 of various kinds are thus laid bare. Basalt is one of these 

 rocks, and it not unfrequently presents itself as a mass of 

 perpendicular six-sided columns, each column 10 ft. or 

 more high, and often a foot or more in diameter. The 

 " Giant's Causeway," in the North of Ireland, and the 

 " Pavee des Geants," in the Ardeche of Southern France, 

 are examples both of which I have visited. It is not easy 



