244 VULCANISM 



action of volcanoes is attributed to the slow feeding of the liquid 

 threads from the middle zone, which is liquefied in spots only. The 

 frequent pauses in volcanic action are assigned to temporary defi- 

 ciencies of supply, and the renewals to the gathering of new supplies 

 after a sufficient lapse of time. The distribution of volcanoes in 

 essentially all latitudes and longitudes is assigned to the general 

 nature of the cause. The special surface distribution is assumed to 

 be influenced, though not altogether controlled, by the favorable or 

 unfavorable conditions for escape of lava to the surface. The per- 

 sistence of volcanic action in time is attributed to the magnitude 

 of the interior source, to its deep-seated position, and to the slowness 

 of conduction of heat from the earth's interior. The force of expul- 

 sion is found in the stress-differences in the interior, particularly 

 the periodic tidal stresses, and in the slow pressure brought to bear 

 on the slender threads of liquid by the creep of the adjacent rock. 

 The violent explosions are due to the included gases, of which steam 

 is chief. Little efficiency is assigned to surface-waters, and that little 

 is regarded as secondary and incidental. The true volcanic gases 

 are regarded as coming from the deep interior, and as being, after 

 expulsion, accessions to the atmosphere and hydrosphere. The 

 standing of the lavas in volcanic ducts for hundreds and even 

 thousands of years with only little outflow, as in some of the best- 

 known volcanoes, is regarded as an exhibition of an approximate 

 equilibrium between the hydrostatic pressure of the deep-pene- 

 trating column of lava and the flowage-tendency of the rock-walls, 

 the outflow being also conditioned on the slow supply below, and 

 on the periodic stress-differences of the interior. 



For the present, volcanic hypotheses must be left to work out 

 their own destiny, serving in the meantime as stimulants of research. 

 All but the last have been long under consideration. The recent 

 discovery of the heating effects of radio-activity has given rise to 

 the hypothesis that the origin of lavas is due to this cause. It 

 seems clear that this must at least be a cooperative agency. It is 

 too early in the new investigation to decide whether it can wisely 

 be regarded as the sole cause or even an essential one. 



How lava reaches the surface. All views that locate the origin 

 of the lavas deep in the earth must face the difficulty of the passage 

 of lava through rock below the fracture zone. Near the surface, 

 the lavas usually take advantage of bedding-planes, or of fissures 

 already existing, or made by themselves. There is little evidence 



