33 
Se eee 
very constant voleano 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 8,000 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 
Stabize, 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, 1 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 afforded 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 
