VULCANISM 391 



earth's interior. The force of expulsion is found in the stress- 

 differences in the interior, particularly the periodic tidal and other 

 astronomic 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 radioactivity 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. 



Modes of Reaching 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 the zone of 

 the earth 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 that they 

 bore their way through the zone of fracture by melting, though 

 they round out their passageways into pipes as they use them, 

 much as streamlets on glaciers falling into crevices round out 

 moulins. But this use of fissures and bedding-planes for passage 

 is probably merely a matter of least resistance where the lavas are 



