VULCANISM 385 



through which the lava passes, and the expansion and escape of 

 the gases. In cases where determination has been practicable, 

 it has been found that the melting-points of silver (about 960 C.) 

 and copper (about 1,060 C.) are reached. From these and other 

 facts it is probably safe to assume that the original temperatures 

 of the lavas as they rise to the surface are sometimes considerably 

 above 2,000 Fahr. (1,093 C.). Even such a temperature must be 

 somewhat below the original temperature of the lava, because some 

 heat must be lost in rising, both by contact with the walls of the 

 colder rocks, and by the expansion of the gases within them. If 

 any considerable part of these gases is derived from waters which 

 join the lava in its upward course, the energy consumed in raising 

 the water to the high temperature of the lavas must be subtracted 

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

 of temperature. It seems probable that temperatures as high as 

 those necessary for ordinary fusion, and perhaps even higher, are 

 attained by the lavas at their sources. 



Depth of the source of lavas. Attempts have been made to 

 determine the depth from which lavas rise, by calculations based on 

 the earthquake tremors that accompany eruptions; but such calcu- 

 lations really tell very little concerning the true point of origin of 

 the lava. At most they probably tell merely 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 be capable of giving rise to seismic tremors. The tremors 

 perhaps compel us to place the beginning of movement of lava at 

 least as low as the bottom of the fracture zone, but they probably offer 

 no sufficient ground for limiting the lava's origin to this or any 

 other specific depth. 



Volcanic Gases 



One of the most distinctive features of volcanoes is the explosive 

 action arising from the gases and vapors pent up in the lava. The 

 precise way in which they are held has not been determined. It has 



