Natural History of Volcanos and Earthquakes. 251 



But if the afflux of water be interrupted by an obstruction of 

 the water-ducts, and if none of the above-mentioned causes be 

 capable of restoring the communication ; or if, during the repose 

 of tlie volcano, the lava-ducts become so obstructed by consolida- 

 tion, that the steam cannot force its way through them, a volcano 

 may reach a state of perpetual repose. Such causes may have 

 effected the extinction of the volcanic activity of the numerous 

 extinct volcanos distributed throughout the globe. If this took 

 place at a former period, when the thickness of the crust of the 

 earth was still increasing considerably, in consequence of the 

 gradual cooling of the earth, and as this process is still going 

 on, there is no probability that such extinct volcanos will at any 

 time become again active. 



If volcanos, for instance Etna, are considerably elevated above 

 the surface of the earth, it commonly happens that the walls of 

 the lava-channels cannot resist the pressure of the melted matter 

 in their interior. In this case rents are formed from which the 

 lava issues. Such rents are always seen in the direction of the 

 axis of the volcanic cone,* and their extent is often very consid- 

 erable. A rent of this description produced by one of the most 

 violent eruptions of Etna, viz. that of the 11th May, 1669, was 

 2^ German miles in length, and occupied almost one-third of its 

 height. Scropef saw distinct traces of it near Nicolosi so late as 

 the year 1819. Even the rent formed during the eruption in 

 1794, on the declivity of Vesuvius, towards Torre del Greco, 

 was, according to Yon Buch 3000 feet in length, and according 

 to Breislak, 237 feet in breadth at its upper edge. 



Other volcanos afford instances of the formation of rents and 

 hills. Thus, during the most violent eruption of Scaptar Jokul 

 on Iceland, in 1783, a rent eight English miles in length was 

 formed in a plain at the foot of the mountain. Three craters, 

 from which immense quantities of lava flowed out, rose in the di- 

 rection of the rent, and afterwards a fourth appeared below the 

 sea in the same direction, and at a distance of thirty miles, the 

 eruption of which formed an island, which afterwards disappeared 

 again.J Similar phenomena took place in the same year in the 

 island of Java. And Yon Buch'§. informs us, that in the island of 



* Von Buch's Beobachtungen, &c., t. ii, p. 137. 



t Considerations, p. 158. X Ibid, p. 154. 



§ Leonhard's Taschenbuch, 1824, t. ii, p. 439. 



