SUBMARINE VENTS 



55 



our Pacific States, such as Mt. Shasta, Mt. Hood, and Mt. Rainier. 

 Most cones are built up of scoriae, ashes, and lava flows, while 

 the fissures that radiate from the crater are filled by dikes, 

 greatly strengthening the mountain, as in the case of Vesuvius. 

 The latter is noted for its double head, Monte Somma being 

 part of the old cone which was mostly blown away in the erup- 

 tion of 79 A.D. 



Fig. 19. — Vesuvius and Monte Somma. 



Volcanoes, like other mountains, are subject to the destructive 

 activity of the atmosphere, of rivers and of the sea, and, when 

 eruptions have ceased, this destruction may go on with great 

 rapidity, especially in the case of cones made up of loose materials. 

 Very ancient cones can seldom be found, for this reason, and often 

 the lava-filled pipe is the only record left of an ancient volcano. 



Submarine Volcanoes have been actually observed in eruption 

 in several instances, but there can be little doubt that by far 

 the larger number have escaped detection. Not a few volcanic 

 islands in various parts of the world have reared themselves 



