136 DYNAMICAL GEOLOGY. 



anese group), in 1815, the explosions are said to have 

 been heard in Ceylon, nine hundred miles distant. The 

 quantity of smoke and ashes was so great that, hanging 

 in the air, they produced absolute darkness for many 

 days, and falling, covered the sea over an area of one 

 hundred miles radius. It has been estimated that the 

 ashes ejected were sufficient to cover the whole of Ger- 

 many two feet deep, and if piled in one place would make 

 a mass three times the bulk of Mont Blanc (Herschel). 



Of lava-eruptions, perhaps the greatest is that of Reyk- 

 janes (Skaptar) in 1783. The mass outpoured has been 

 estimated as twenty-one cubic miles (Herschel). These, 

 however, are extreme cases. One of the greatest erup- 

 tions of Kilauea, that of 1840, poured out a lava-stream 

 forty miles long, which, if accumulated in one place, 

 would cover an area of a square mile eight hundred feet 

 deep. The average of lava-flows, however, is far less. 

 One of the greatest eruptions of Vesuvius poured out 

 600,000,000 cubic feet of lava. This would cover a 

 square mile twenty-two feet deep, or would make a 

 stream seven miles long, one mile wide, and three feet 

 thick. 



Moiiticles. In volcanoes of moderate height, eruptions 

 usually come from the top of the cone or principal crater, 

 but in very lofty volcanoes the pressure necessary to raise 

 lava so high fissures the mountain in a radiating manner. 

 These fissures are filled with liquid matter, which, on 

 hardening, form radiating dikes. Eruptions often take 

 place through these fissures, and thus form subordinate 

 craters and cones about the main cone ; these are called 

 monticles. About six hundred such monticles dot the 

 surface of Mount Etna, some of which are seven hundred 

 feet high above the level of the mountain-slope on which 

 they stand. About Mount Shasta (which is a recently 

 extinct volcano) ;ire found ;i number of these monticles. 



Nature of the Materials Erupted. The materials 



