VOLCANIC ISLANDS. 



11. 



Fig. 195. Fig. 196. 



Appearance and form of certain volcanic islands. 



an annular rock, almost on a level with the surface of the sea, in 

 the midst of which was a lagune having a single outlet, and in 

 which the water was burning. Now, these islands appear to be 

 nothing more than points of domes upheaved, like those in the 

 gulf of Santorin, either instantaneously or slowly, and having the 

 summit broken, like Monte-Nuovo. These are true craters of 

 elevation or of explosion, as we would call them ; and as such 

 they may consist of solid rocks, or of various tufas, or even of 

 scoriae accumulated on their borders. The archipelago of the 

 Azores, which have so often witnessed rising from the sea similar 

 islands, which time has destroyed, presents us one which seems to 

 have escaped destruction, to exhibit to us how all those were 

 formed which have disappeared. This is the rock of Porto de 

 Ilheo, which presents a vast circle, into which vessels enter for 

 shelter; its sides rise 400 feet and are composed of volcanic tufa. 



19. These phenomena explain to us the origin of a great many 

 islands found in the ocean (fig. 197), both by the analogy of their 

 form to those we have named, and their nature. Some are in the 



form of a horse-shoe, having a more ._ - 



or less expanded opening, which rr~~~^~^ 

 gives access to the middle of the 



o 







deep basin they enclose, and in the 

 centre of which isolated volcanic 

 hillocks are occasionally found. Fig. 197. Disposition of certain 

 Others are entirely circular, having islands in the South Scis 

 some of the points of the circle more or less broken, or groups of 

 small islands arranged in a circle, which are more or less promi- 

 nent above the water. 



20. Different periols of the formation of a volcan. We may 

 often distinguish in the mass of a volcanic mountain, several dif- 



19. How do volcanic phenomena explain the origin of certain islands 7 



