FROZEN SEAS OF LAVA. 91 



with a pipe leading down into the earth. Through this the 

 lava rises into the cavity. When the strains have sufficiently 

 accumulated, the lava is forced above its usual level some- 

 times overflowing the lips pf the crater; sometimes bursting 

 the walls of the mountain, thinned by melting from within. 

 Sometimes, also, the walls by internal fusion become so much 

 weakened that the whole summit falls in, leaving an enor- 

 mous open chasm. Over this a solid crust forms by exposure. 

 Then, in subsequent ages, this is pierced by a new rupture, 

 around which a new, and smaller, cone is built up, with the 

 broken margin of the older one still more or less perfectly 

 preserved. So, during the eruption of 79, the crater of Ve- 

 suvius collapsed, and the present crater has since grown up, 

 leaving still on the north a vast rampart, Somma, showing 

 where the line of rupture of the ancient cone was traced. 



XVI. KRONEN SEAS OR 



ANCIENT LAVAS. 



THE spectacle of a volcano in a state of active eruption is 

 a terrific demonstration of the forces of fire imprisoned within 

 the earth, and escaping to our view only when their accumu- 

 lated strength exceeds that of the restraints in which they are 

 held. These are activities of the present ages of the world, 

 and proofs of intense heat now existing within the cool ex- 

 terior. Geology brings to our notice the records of still 

 vaster and more terrific operations of intense heat. Vast as 

 are the volumes of modern eruptions, they are slight com- 

 pared with eruptions of former geologic ages. The limited 

 amount of matter poured forth in modern times cools near the 

 place of escape, and seldom flows to the distance of ten miles. 

 It accumulates, therefore, around the vent, and builds up a 

 volcanic cone.^ In earlier times the molten lava issued in such 

 quantity as to retain its liquid state sufficiently long to flow 

 away sometimes a hundred miles or more, and overspread 

 with a sea of fire regions as broad as States. The modern 



