no 



VOLCANIC ISLAND*. 



rounded by elevated rocks, having a single opening only for 

 entrance &c. 



Islands which have been elevated in the midst of the sea frequently 

 exhibit phenomena of the same kind. Thus the islands of Santoriri, The- 

 rtsia, Aspronisi, (Jig. 193), which were elevated long before the Christian 

 era, present the appearance of a vast crater of elevation : their slopes are 

 gentle (Jig. 193} externally, but abrupt, on the contrary, towards the centre 



Theresia. 



Santorii). 



193. Section of Santorin and adjacent islands. 



of the circle of which they form the margin. The ground is composed ol 

 various stratn, inclined outwardly, among which are limestone and argilla' 

 ceous schist. In the middle of the circle, the depth of which is considera- 

 ble on the borders, all the subsequent volcanic phenomena were produced, 

 and here the three summits of cones successively appeared, which consti- 

 lute three modern islands, and are still prepiring new eruptions. 



Something similar is seen in the Gulf of Bengal, on the Island of Barren, 

 discovered in 1787. It is a vast circle (Jiff. 194) formed of high moun- 

 tains, into which the sea penetrates by a single opening, arid has a volcan 

 in the centre which was in full activity at the time of the discovery. 



Fig. 194. Vie^of the Island of Barren in the Gulf of Bengal. 



18. Similarity of configuration in Volcanic Islands. DifTeren' 

 volcanic islands which have been formed under our eyes, as it were, 

 in the midst of the ocean, are entirely analogous to those we have 

 mentioned. The island of Sabrina, at the moment of its appear- 

 ance, presented a crater which opened to the south, (Jigs. 195, 1 ( J6). 

 and terminated by an opening, through which issued a current of 

 boiling water: according to the accounts, the island of Julia must 

 have been somewhat analogous ; and the history given by Captain 

 Tha) T er, reported by Poeppig, shows such to have been the case. 

 On the 6th September, 1835, to the north of New Zealand, this 

 navigator almost witnessed a submarine eruption, which presented 



18. How do volcanic islands differ from each other in form? 



