THE EXTRUSIVE PROCESSES. 631 



buted to the magnitude of the interior source^ to its deep-seated location, 

 and to the slo\\Tiess of conduction of heat in the earth's interior. The 

 force of expulsion is found in the stress-differences in the interior, par- 

 ticularly the periodic tidal and other astronomic stresses (see p. 580), and 

 in the slow pressure brought to bear on the slender threads of hquid by 

 the creep of the adjacent rock. The violent expulsions are due to the 

 included gases, of which steam is chief. Little efficiency is assigned to 

 surface-waters, and that little is regarded as wholly secondary and inci- 

 dental. The true volcanic gases are regarded as coming from the deep 

 interior and as being true accessions to the atmosphere and hydrosphere. 

 The standing of the lavas in volcanic ducts for hundreds and even 

 thousands of years with only small outflows, as in some of the best-kno^\Ti 

 volcanoes, is regarded as an exhibition of an approximate equihbrium 

 between the hydrostatic pressure of the deep-penetrating column of 

 lava, and the flowage-tendency of the rock-walls, the outflow being, of 

 course, also conditioned on the slow rate of supply below, and the peri- 

 odic stress-differences of the interior. 



For the present these hypotheses must be left to work out their 

 o^^m destiny, serving in the mean time as stimulants of research. All 

 but the last have been for some time under the consideration of geolo- 

 gists, and are set forth in the literature of the subject (p. 636). 



A few special phases of the problem need further discussion, though 

 they have been incidentally touched upon. 



Modes of Reaching the Surface. 



All of the views that locate the origin of the lavas deep in the earth 

 must face the difficulties of the passage through the dense portion of 

 the sphere below the fracture zone. Near the surface, the lavas usually 

 take advantage of fissures or bedding-planes already existing or made 

 by themselves. There is little CA^dence that they bore their way by 

 melting, though they round out their ducts 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 relatively cool, 

 and their capacity for melting is low or perhaps even gone. Daly has 

 recently urged that lavas work out reservoirs and enlarge passageways 

 for themselves by detaching masses of rock from the roofs and sides 

 of the spaces already occupied by them, these masses either melting 



