VOLCANIC ISLANDS. 



A ground-plan of the Yal del Bove and the surrounding 

 parts of the mountain is shown in fig. 63. 



Lava of 1832 



Terminal cone 

 Val del Bove 



Lava of 1600 

 Catania 



Cycloptean isles 



Taormiuiu 



Fig. 63. Plan of the Val del Bove, Mount Etna. 



148. The sudden sinking of the ground by which these crater- 

 formed cavities are produced, is often attended with the sudden 

 production of lakes, filling such cavities with water from sub- 

 terranean sources. Such lakes are sometimes supplied with water 

 at a high temperature, as was the case in one produced in 1835 

 in Cappadocia, near the ancient Ceesarea, and in 1820 in the 

 island of St. Michael, one of the Azores. 



149. Volcanic islands generally are found to affect the semi- 

 lunar form, of which Santorin, fig. 58, is an example. Thus the 

 island of Sabrina, already mentioned, which appeared in 1811 

 among the Azores, at the moment of its rise above the waters 

 presented a crater which opened towards the south and was ter- 

 minated by crevasses, or openings, from which issued a current of 

 boiling water, figs. 64 and 65. 



The island of Julia, which appeared to the south-west of Sicily 

 in 1831, assumed a similar form, and on the 6th of September, 

 1835, Captain Thayer, the French navigator, found to the north 

 of New Zealand a similar rock recently formed near the surface 

 of the water, which included a lagoon, having a single issue, and 

 within which the water was boiling. 



150. These craters of elevation have sometimes continued per- 

 manently in the form which they first assumed, but they have 

 also frequently been subject to subsequent changes of form from 

 age to age. The case of Vesuvius, which underwent a remarkable 

 change in 79 A.D., has been already mentioned. The Peak of 

 Teneriffe rises within a circular enclosure, the sides of which are 

 vertical, and rise to a height of from twelve hundred to two 



111 



