But with the help of this outflow the thickness of the crust of the island increased materially, and therefore also its powers of resistance. It may easily be conceived what vast quantities of elastic gases, what masses of molten matter accumulated beneath its solid surface whilst no exit was practicable after the cooling of the trachytic crust. Therefore a time would come when the elastic and explosive forces of the imprisoned gases would upheave this ponderous cover and drive out for themselves openings through tall chimneys. Hence then the volcano would distend and lift up the crust, and then burst through a crater suddenly formed at the summit or thinnest part of the volcano.

To the eruption succeeded other volcanic phenomena. Through the outlets now made first escaped the ejected basalt of which the plain we had just left presented such marvellous specimens. We were moving over grey rocks of dense and massive formation, which in cooling had formed into hexagonal prisms. Everywhere around us we saw truncated cones, formerly so many fiery mouths.