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 the volcano, the lava-ducts become so obstructed by consolida- 
tion, that the steam cannot force its way through them, a voleano 
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 Hina, 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 Hina, viz. that of the 11th May, 1669, was 
2 German miles in length, and occupied almost one-third of its 
height. Scrope+ 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 Von Buch 3000 feet in length, and according 
'o 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 Seaptar Jokul 
°n 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- 
fection of the rent, and afterwards a fourth appeared below the 
Sea in the same direction, and at a distance of thirty miles, the 
*uption of which formed an island, which afterwards disappeared 
‘gaint Similar phenomena took place in the same year in the 
island of Java, And Von Buch informs us, that in the island of 
Saar ce eae NP eta Ie a MES OO ee de Oe 
* 
Vv : 
t Considerations, . 158. t Ibid, p. 154. 
§ Leonhard’s Taschenbuch, 1824, t. ii, p. 439. 
