Geology and Mineralogy. 395 



descended from breaks in its sides. The appearances indicate 

 comparatively recent action, perhaps within a century. The 

 inner cone, according to Mr. Mallet, has certainly been made 

 within the last 1800 years. No evidence of the asserted presence 

 of the sea at any time within the limits of the amphitheater was 

 found. 



The heat below still gives rise to fumaroles and hot springs. 

 Notwithstanding the "heavy tropical rains," the island has no 

 surface streamlets, all the waters becoming subterranean. The 

 water of the principal hot springs is brackish — containing over 

 200 grains of saline matter to the gallon. 



The lava is mostly doleritic or basaltic, and much of it contains 

 chrysolite. The formation of the amphitheater and the break in 

 it on the west side are attributed to a " blowing away of the upper 

 and central part of the cone by a great explosive eruption," like 

 that of Krakatoa, Aug. 27, 1883. 



The expression explosive eruption, which is above used and is 

 common in accounts of violent volcanic action, is misleading. It 

 seems to imply that in such eruptions, the top of the lava-made 

 cone is projected off in the explosion, and that by this means 

 came the great crater-area and its encircling walls; or that the 

 ejections upward from the vent are the chief means of making the 

 great cavity. But no one has yet reported the occurrence of 

 large masses or slabs of the once stratified lavas from the lost 

 part of the cone scattered over the remaining outside slopes, or 

 shown that the outflow of lavas at the eruption may not have 

 great undermining action. 



Volcanic action in a crater, where there are liquid lavas, is 

 largely a pseudo-boiling process ; and the confined vapors of the 

 enlarging bubbles, forcing an escape, are the chief projectile 

 agency through which the more terrific explosive phenomena are 

 produced, as well as the gentler of Kilauea-like type. Aug- 

 mented activity means an augmented and more rapid escape of 

 vapoi-s through the viscid lavas ; it means also augmented heat 

 and a consequent extension of the region of liquid lavas, at and 

 below the surface, by fusion of the adjoining solid lavas, and 

 hence augmentation in explosive action. Cinder cones made 

 about the vent in the quieter times may be destroyed by the 

 projectile violence or become engulfed in the spreading region of 

 fusion ; and the rocks of the crater throat may be to some extent 

 torn off and added to the ejectamenta. Finally, in the catas- 

 trophic eruption when the force from the rising vapors and from 

 other conditions becomes greater than the mountain can with- 

 stand — a point often abruptly reached — the sides break and one 

 or more fissures let out the liquid lavas. However explosive the 

 action, the solid rock of the summit of the cone, while it may be 

 more or less removed by the forces engaged, instead of being 

 projected over the outer slopes, sinks down into the abyss so 

 made. Thus a volcanic cone under the most formidable of explo- 



