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very constant volcano is Sangy, 17,128 feet above sea level in the 

 Cordileras. This indefatigable mountain has, according to Sebas- 

 tian Wiss, no less than 269 eruptions every hour, or about one 

 every thirteen seconds (Humbolt). It is often a very difficult 

 matter to say whether a volcano is extinct, or merely dormant, for 

 the 2,000 or 3,000 years of which alone we have any record or 

 tradition count for so little in geological reckoning, that it by no 

 means follows that a crater is extinct because no record exists of 

 any eruption from it. So that it would not be at all surprising if 

 some of our so-called extinct volcanoes were, any day, to break 

 forth into renewed activity, say at Auvergne (in central France) 

 for example. There was not even a tradition of volcanic action 

 connected with Tomboro when it suddenly woke from its sleep of 

 centuries into wild and disastrous eruptions. In Nero's time 

 Vesuvius was considered an extinct volcano, its sides were highly 

 cultivated and covered with flourishing vineyards and an industrious 

 population. It was then an obtusely topped mountain, over 4,000 

 feet high, with a shallow depression, the old crater, a mile and a 

 half broad, covered with greenery, and overgrown with wild vines, 

 and apparently as peaceful as Sugar Loaf Hill. It was in this 

 picturesque crater that Spartacus the Thracian encamped with his 

 heroic band of gladiators during the servile war. After a premoni- 

 tory series of earthquakes, which lasted some ten years, Vesuvius 

 suddenly woke into furious activity. The ancient summit, the 

 remains of which are now called Monte Somma, was totally dis- 

 rupted and blown into the air, and Pompeii, Herculaneum, and 

 Stabiffi, were buried in the dust and debris. Some of this volcanic 

 dust is so fine that Professor Bonney has calculated it would take 

 25,000 particles to weigh a single grain, and it is so light that it 

 will remain for many days in the air, and is borne by the wind for 

 hundreds of miles. Since that memorable eruption in A.D. 79, 

 when the present peak of Vesuvius was first formed, its shape has 

 frequently altered, being by some eruptions piled up, and by others 

 lowered several hundreds of feet. Lava rapidly cools on the 

 surface, and it is even possible to walk over slowly flowing floods 

 of it without burning the feet. As a curious illustration of what a 

 bad conductor of caloric it is, I may mention that Sir Chas. Lyell 

 relates having seen a large glacier of ice, on the flank of Mount 

 Etna, entirely covered, first with a large layer of fine volcanic sand, 

 and then a sheet of lava, which must have flowed over it, and 

 paradoxical as it sounds, preserved it from melting. It had re- 

 mained unmelted for at least 30 or 40 years, and afibrded a supply 

 of ice to the city of Catania for a number of years. Although 

 volcanoes generally follow the coast lines, the pent-up forces some- 

 times find a weak spot in the ocean bed, and then ensues a fierce 



