36 VOLCANOES 



and others are those on the east and west coasts of Africa and in 

 the Indian Ocean. The prevalent trend of the belts is thus in a 

 generally north and south direction. 



A very striking fact is the nearness of almost all active volcanoes 

 to the sea ; by far the greater number of vents are upon islands, 

 and those of the continents are, with few exceptions, not far from 

 the coasts. Another relation which should be noted, is that 

 between the volcanic bands and the mountain chains, the bands 

 running parallel to or coinciding with the mountains, as in the 

 great volcanoes of the Andes. Not all coast lines or all moun- 

 tain chains have volcanoes associated with them, but where the 

 mountains are near the seashore, volcanoes are usually, though 

 not invariably, found. The seat of volcanic activity is frequently 

 shifted, as we have learned, and it has been observed that this 

 activity tends to die out of the older rocks and to make its appear- 

 ance in those of a later date. 



Volcanic Eruptions. — The phenomena displayed by different 

 volcanoes, or even by the same volcano at different times, vary 

 greatly. It often seems difficult to believe that similar forces are 

 involved, and that the divergences are due merely to different cir- 

 cumstances attending the "outbreak. A careful comparison, how- 

 ever, of the varying phenomena brings to light a fundamental like- 

 ness in them all. Some vents, like Stromboli in the Mediterranean, 

 are in an almost continual state of eruption of a quiet kind ; others, 

 like Vesuvius, have long periods of dormancy, broken by eruptions 

 of terrible violence. In a general way, it may be said that the 

 longer the period of quiet, the more violent and long-continued 

 will the subsequent eruption be, while weak eruptions and those 

 of short duration recur at brief intervals. 



The comparatively gentle operations of Stromboli give the ob- 

 server an opportunity to learn what are the essential phenomena 

 of a volcanic eruption. Though occasionally breaking out with 

 violence, Stromboli is for long periods in such exact equilibrium 

 that barometric changes have a marked effect upon its activity, 

 and the Mediterranean sailors make use of it as a weather-signal. 

 The floor of the crater is formed by hardened lava, the cracks in 



