CHAP. VI MOUNTAINS 243 



Though long extinct, volcanoes once existed 

 in the British Isles ; Arthur's Seat, near 

 Edinburgh, for instance, appears to be the 

 funnel of a small volcano, belonging to the 

 Carboniferous period. 



The summit of a volcanic mountain is 

 sometimes entirely blown away. Between 

 my first two visits to Vesuvius 200 feet of the 

 mountain had thus disappeared. Vesuvius 

 itself stands in a more ancient crater, part 

 of which still remains, and is now known as 

 Somma, the greater portion having disap- 

 peared in the great eruption of 79, when the 

 mountain, waking from its long sleep, de- 

 stroyed Herculaneum and Pompeii. 



As regards the origin of volcanoes there 

 have been two main theories. Impressed by 

 the magnitude and grandeur of the phenom- 

 ena, enhanced as they are by their destruc- 

 tive character, many have been disposed to 

 regard the craters of volcanoes as gigantic 

 chimneys, passing right through the solid 

 crust of the globe, and communicating with 

 a central fire. Recent researches, however, 

 have indicated that, grand and imposing as 



