VULCANISM 181 



miles. Such a temperature would be enough to liquify rocks at 

 the surface; but it does not follow that they would be liquid at the 

 depth of 50 miles at this temperature. At this depth, the pressure 

 of the overlying rock is enormous, probably enough to keep the 

 rock solid long after it reached a temperature which would make it 

 liquid at the surface. There are other reasons for believing that, 

 though the temperature of the interior of the earth is very high, 

 the rock is still solid. 



Among the vapors which escape from volcanoes, there are 

 those which might have been derived from sea-water. From this 

 fact it was formerly thought that sea-water had access to the sources 

 of the lava. It is now believed, however, that water from the sur- 

 face does not descend more than five or six miles (p. 30). It seems 

 certain that the sources of the lava are much deeper, and it therefore 

 seems improbable that descending water, either from the sea or 

 from the land, reaches these sources. 



Since lava is formed at depths and at temperatures which can 

 not be studied, there are many points concerning its origin about 

 which we cannot be sure. But it seems probable (1) that the lava 

 is being formed all the time, in spots, in the deep interior, and (2) 

 that it is all the time finding its way to the surface, but faster and 

 in greater quantities at some times than at others. The regions 

 where the crust is least stable, that is, where there is movement, 

 are the regions most likely to afford the lava a place of escape. 



REFERENCES 



1. Standard text-books on Geology and Physical Geography. 



2. RUSSELL, Volcanoes of North America: Macmillan. 



3. JUDD, Volcanoes: Appleton. 



4. BONNEY, Volcanoes: Putnam. 



5. Recent Eruptions in the West Indies. HEILPRIN, Mt. Pelee and the 

 Tragedy of Martinique, Lippincott; HILL, Nat. Geog. Mag., Vol. XIII, pp. 

 223-267; RUSSELL, Nat. Geog. Mag., Vol. XIII, pp. 267-285, 415-435; 

 HOVEY, Nat. Geog. Mag., Vol. XIII, pp. 444-459, Am. Jour. Sci., Vol. XIV, 

 1902, pp. 319-358. and BULL. Am. Mus. Nat. Hist., Vol. XVI, pp. 333-372. 



6. The recent eruption of Vesuvius, JAGGAR, Nat. Geog. Mag., Vol. XVIII, 

 p. 318; HOBBS, Jour, of Geol., Vol. XIV, p. 636. 



7. DILLER, Mt. Shasta, in Physiography of the United States: Am. Book Co. 



