212 J. LOGAN LOBLEY^ F.G.S.^ F.R.G.S.^ ON VOLCANIC ACTION 



The material ejected and shot high up into the air — heavy 

 masses, rounded " bombs," cindry fragments or scoriae most 

 irregular in form and partly vesicular, lapilli, and fine dust or 

 ash — is often so great as to quite take away the light of the 

 day, and the finer particles ascend to great heights and are 

 then carried by winds and upper currents of the atmosphere 

 to long and, in some cases, immense distances. Vast volumes 

 of steam are given off, which condensing, form with the ash a 

 mud, often wrongly called lava, that sometimes rolls down the 

 slopes to the base of the volcano in a destructive torrent. 

 Sea waves of great magnitude are also sometimes produced 

 by displacement, or movement, of adjacent sea-bottoms and 

 land masses either insular or coastal, which may occasion 

 great destruction to life and property. These destructive 

 oceanic waves are invariably wrongly called "tidal waves" 

 by the newspapers, although they are seismic waves, and 

 have nothing whatever to do with the tides, which are periodic. 

 This is an illustration of the little attention paid to even the 

 most elementary science in this country in ordinary education. 

 Electrical phenomena are also produced, for volcanic lightning 

 plays amongst the ascending ash-charged fumes. 



The great historical eruption of Vesuvius, in a.d. 79, was 

 an eruption of this class. By it the cities of Pompeii, 

 Herculaneum, and Stabite were destroyed, and during its 

 continuance the darkness was complete. Dry lapilli and ash 

 overwhelmed Pompeii and Stabise, both at considerable distances 

 from the crater, while a torrent of mud overwhelmed the city 

 of Herculaneum, immediately at the foot of the mountain 

 slopes, and no lava anywhere issued from the volcano. For 

 a long period it would appear that the Vesuvian eruptions 

 were explosive eruptions only, though of much less violence, 

 since it was not until A.D. 1036 that there was any record of 

 a lava-flow there, although in pre-historic eruptions, as shown 

 by the basaltic rocks of Monte Somma, lava was abundantly 

 emitted by this vent. 



The Krakatoa eruption of 1883 is the greatest recent 

 example of a purely explosive eruption, and by it the island 

 of Krakatoa was almost destroyed. It produced complete 

 darkness, and ejected by its explosive force the material of 

 two-thirds of an island of thirteen square miles, covering the 

 adjacent seas with floating lapilli of pumice. Its fine ash w^as 

 carried upwards to a height estimated at 50,000 feet, the 

 finest and highest having been carried three times round the 

 globe, and occasioning the very beautiful sunsets of that time, 



