VOLCANOES 



at twelve noon, we are to be aboard the Australia, 

 and by the 7th we expect to reach San Francisco, the 

 post-office of which city will dispatch this letter east- 

 ward." 



Dana's mature reflections shall be given in his own 

 words from the preface to the volume on Volcanoes, 

 already cited : 



" Science has learned from Hawaii more than it knew 

 of the mobility of liquid basalt ; of the consequent range 

 in flow-angle of basalt-lavas, from the lower limit near 

 horizontality to the verticality of a waterfall, and there- 

 fore of lava-cones of the lowest angle, and driblet-cones 

 of all angles ; of lava-lakes tossing up jets over their fiery 

 surface like the jets of ebullition, and in other cases play- 

 ing grandly in fountains hundreds of yards in height; 

 and, consequently, of the absence from the craters of 

 large cinder-ejections. It has further learned of a degree 

 of system in the changes within a crater from one epoch 

 of eruption to a state of readiness for another; of a sub- 

 sidence, after an eruptive discharge of lava, that has 

 carried down, hundreds of feet, a large part of a crater's 

 floor without a loss of level in its surface; and, following 

 this, of a slow rising of the subsided floor, chiefly through 

 the ascensive or upthrust action of the lavas of the lava- 

 column, and the lifting force taking advantage of the 

 fault-planes that were made at the subsidence ; and also 

 of debris-ridges and debris-cones, one to two hundred 

 feet in elevation, made, by the lift, out of the talus of the 

 pit-walls. 



11 It has learned that pit-shaped craters are characteris- 

 tic of true basalt volcanoes, and a result of the free mobil- 

 ity of the lavas, whether the action in the lava-lakes 

 within be fountain-like or boiling-like ; that floating 

 islands of solid lava may exist in the lakes ; that a regular 

 oscillation between fusion and cooling takes place at times 

 in the thin crust of lava-lakes ; that the solid lava of the 

 margin of a lake may be re-fused, and also even the mass 

 of a floating island, and the blocks of a debris-cone until 

 the cone has wholly disappeared. 



" It has discovered that solfataric action, or that of 



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