VOLCANOES. 89 



canoes of France and Germany are in tertiary regions. Possibly the 

 retiring of the sea has extinguished them. In the oldest strata vol- 

 canic activity has apparently died out long ago. 



Phenomena of an Eruption. — The phenomena of an eruption are very 

 diverse. Sometimes there is a gradual melting of the floor of the crater, 

 and then a rising and boiling of the liquid contents until they quietly 

 overflow and form immense streams of lava, extending fifty to sixty 

 miles. After the eruption, the melted lava again sinks and cools, and 

 solidifies, to form the floor of the crater until another eruption. This 

 is the case with the Hawaiian and many other volcanoes in the South 

 Seas. In other cases, as in the Mediterranean volcanoes, and especially 

 in many in the Indian Ocean, the eruption is fearfully explosive. In 

 such cases the eruption is usually preceded by premonitory earthquakes 

 and sounds of subterranean explosions ; then the bottom of the crater 

 is blown out with a violent explosion, throwing huge rocky fragments 

 to great distances, often many miles ; then the melted lava rises and 

 overflows in streams running down the side of the mountain. The rise 

 and overflow of lava are accompanied with violent explosions of gas 

 which throw up immense quantities of ashes and cinders 6,000 and 

 even 10,000 feet above the crater.* The fine ashes from Krakatoa are 

 said to have been carried, by the uprush of gas and vapors, to the 

 amazing height of 17 miles, f In the great eruption of Tomboro, in 

 the island of Sumbawa near Java, in 1815, these explosions were heard 

 in Sumatra, 970 miles distant. \ Explosions of Krakatoa were heard 

 2,000 and even 3,000 miles.* The emission of gas usually continues 

 after all other ejections cease. Violent storms and heavy rain accom- 

 pany the eruption, and when the mountain reaches into the region of 

 perpetual snow, as in many of the South American volcanoes, the 

 fearful deluges produced by the sudden melting of the snows are often 

 the most destructive phenomenon connected with the eruption. 



Volcanic eruptions, therefore, may be divided into two great types, 

 viz., the quiet and the explosive. In the one, lava-flows predominate ; 

 in the other, cinders and ashes, and steam and gas. The Hawaiian vol- 

 canoes are perhaps the best examples of the former, and the Javanese vol- 

 canoes, especially Krakatoa, of the latter. The Mediterranean and most 

 other volcanoes are mixtures of these two types in varying proportions. 



The quantity of materials ejected during an eruption is sometimes 

 almost inconceivable. During the great eruption of Tomboro, already 

 mentioned, ashes and cinders were ejected sufficient to make three 

 Mont Blancs, or to cover the whole of Germany two feet deep. || In 



" Dana's Manual, p. 692. # Science, vol. iv, p. 134, 1884. 



+ Judd, Nature, vol. xxxviii, p. 540, 1888. | Herschel, Physical Geology, p. 111. 



% Lyell, Principles of Geology. 



