EARTHQUAKES AND VOLCANOES. 



The mud which is ejected from these volcanoes is cold. In a 

 similar mud volcano mentioned by Humboldt, at Damak, in the 

 province of Samarang, in the island of Java, the mud ejected has 

 a high temperature. 



The gaseous eruptions from these salses are usually attended by 

 noise, and consist of different sorts of gas, sometimes hydrogen, 

 sometimes carbonic acid, and occasionally nitrogen. The 

 hydrogen is often mixed with naphtha. 



56. After the first imposing phenomena such as those above 

 described in the case of ^Jokmali have ceased, the mud volcanoes 

 in general seem to be the result of a feeble activity of the interior 

 forces of the globe obstructed in their effects by some impediments 

 in the fissures or openings by which communication with the 

 surface is obtained. The coldness of the mud seems to prove that 

 the seat of the force is not at any great depth. 



57. From the examples of subterranean activity presented by 

 superficial convulsions, earthquakes, thermal springs, and jets of 

 gas and steam, we pass to the formation of volcanoes properly so 

 called. The internal forces, acting with unequal effect on different 

 parts of the solid crust of the earth, surmount its resistance at 

 points where it has least tenacity, and upheaving the incumbent 

 strata, raise them into dome-shaped masses, like those of the Puy 

 de Dome and Chimborazo, without, however, E producing actual 

 fracture. Sometimes the mass thus upheaved gives way at the 

 summit of the dome, which separates so as to leave a circular 

 cavity of a certain depth surrounded by a nearly perpendicular 

 wall, having on the exterior a gradual slope, which formed the 

 declivity of the dome before the disruption. 



58. The roundish cavity thus formed is called a " crater of 

 elevation." 



59. If the energy of the subterranean forces be sufficiently 

 intense, the floor of this crater will be disrupted, holes and fissures 

 will be formed in it, communicating with the liquid fire which 

 fills the solid shell of the earth, steam and acid gases will be 

 ejected in vast quantities, followed by ignited scoriae, and red hot 

 .stones, and fragments of rock, after which will follow torrents of 

 that incandescent earthy matter in a state of pasty fusion, which 

 has been called LAVA; in a word, an active VOLCANO will be 

 formed. 



60. Now there are here several distinct stages, at any one of 

 which the phenomena may be brought to a close, according to the 

 relation between the energy of the upheaving force and the local 

 tenacity of the earth's solid crust. If the upheaving force do not 

 much exceed that tenacity it may spend its entire energy in 

 producing swelling of the surface of the ground more or less pro- 



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