264- Volcanoes. 



The phenomena which accompany eruption are the same 

 in the phase of moderate activity and of prolonged intermit- 

 tences. But in the former they are much less violent ; all the 

 terrible effects of eruption being felt in the immediate neigh- 

 bourhood, but none of its phenomena w^itnessed beyond the 

 narrovvr sphere of its activity. On the other hand, when a 

 volcano is in the phase of prolonged intermittences, the erup- 

 tions are of great violence, and extensive in their effects. The 

 explosions of Cotopaxi are sometimes heard at a distance of 

 600 miles ; and those of Jumbawa, a Molucca island, were 

 heard 900 miles off during the eruption in April, 1815. 



The eruption of a volcano generally commences with a tre- 

 mendous explosion, which is succeeded by others less loud, 

 and the escape of aeriform fluids. Large fragments of rock 

 and masses of lava are projected by these discharges, some of 

 which fall into the crater to be again discharged ; until they 

 are so reduced as to be mingled with the surrounding atmo- 

 sphere of heated vapour. The atomic particles produce the 

 appearance of dense clouds of smoke, which are almost inva- 

 riably seen to surround the summit of the crater. 



The lava then rises to the vent of the mountain ; and soon 

 finds egress, whether it be from the crater or a lateral open- 

 ing. In some cases, however, scoriae alone are projected, or 

 even mud. During the day, the lava is generally hidden by 

 the aqueous vapours that arise from it, and, when observed, 

 does not present the appearance of a heated mass ; but during 

 the night it appears of a glowing heat. While the lava con- 

 tinues to flow, the detonations are frequently less violent ; but 

 it is not until it is entirely stayed that there is any proof of 

 the diminution of the paroxysm : and even the cessation of 

 the lava does not indicate an immediate restoration of peace ; 

 for the ejection of scoriae and masses of rock often recom- 

 mences after the eruption of lava, and continues for some time 

 after the dreadful crisis is past. 



When the detonations become less frequent, rumbling 

 sounds are heard, as of the retreat of many waters ; and the 

 mountain seems gradually to yield to exhaustion, or sinks into 

 a state of partial rest, occasionally disturbed by explosions, 

 the ejection of scoriae and small masses of rock. 



Towards the conclusion of an eruption, or after the lava 

 has ceased to flow, the surrounding country is frequently 

 enveloped in dark clouds of black-coloured sand, which, at 

 the very close of the convulsion, is changed into a white 

 comminuted pumice. 



The lofty mountains seldom eject lava from their summits, 

 but from lateral openings ; for it requires far less power to 



