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GEOLOGY. 



least, and partly from the expansion of the gases within them. If any 

 considerable part of these gases is derived from waters which joined 

 the lava in its upward course in the fracture zone, the energy consumed 

 in raising the water to the high temperatures of the lavas must be sub- 

 tracted from the original heat, and must be a further source of reduction 

 of temperature. It is important to emphasize this point in view of its 

 bearing upon the origin of the lavas. It has been suggested that lavas 

 may be due to an aqueo-igneous fusion, a kind of fusion which may take 

 place at comparatively moderate temperatures. It seems obvious, 

 however, from the phenomena themselves, that temperatures as high 

 as ordinary dry fusion, and perhaps even higher, are attained. It is 

 clear also that the maintenance of the liquid condition in a constant 

 state of ebullition for a long period of time implies a large surplus of 

 heat above that necessary for liquefaction simply. This is especially 

 true if the ebullition comes from surface-waters penetrating to and 

 becoming absorbed in the lava-column below. This process must tend 

 rapidly to exhaust the heat in the column of lava. If, on the other 

 hand, the gases are derived from the deep interior, and the ebullition at 

 the surface is due to their escape, they may bring up new supplies of 

 heat to counteract the cooling effects of their expansion. 



Depth of source. — Attempts have been made to ascertain the depth 

 from which lavas rise, by means of the earthquake tremors that accom- 

 pany eruptions. The estimates have ranged from seven or eight to 

 thirty miles. The mode of estimate is that discussed under earthquakes, 

 and is subject to the corrections there indicated. If these could be 

 perfectly applied, the estimates might probably all fall within ten miles, 

 and not improbably all within six miles of the surface. But in any 

 case the method really tells very little as to the true point of origin of 

 the lava. At most it probably only tells where the ascending lava 

 begins to rupture the rock through which it passes, and rupture may 

 not be possible below the zone of fracture, which is probably not more 

 than six miles deep. In the zone of flowage below, where the pressure 

 is too great to permit fracture, the lava not improbably makes its way 

 by some boring or fluxing process, which might not, because of its 

 nature, be capable of giving rise to seismic tremors. The behavior of the 

 tremors perhaps forces us to locate the origin of lava movement at least 

 as low as the bottom of the fracture zone, but it probably offers no 

 sufficient ground for limiting the lava's origin to this or any other specific 

 depth. 



