THE EXTRUSIVE PROCESSES. 627 



to certain portions only. The great pressures in gas- and oil-wells show 

 that in many quite superficial beds, even when arched, there are no 

 fissures or pores capable of letting even gas escape effectively. The 

 depths at which the temperatures of lavas are reached are usually 

 estimated, from the downward increase of temperature, at 20 to 30 miles. 

 This leaves from 14 to 24 miles of the compressed zone between the 

 lowest assignable limit of the fissured zone, and the highest assignable 

 zone for the origin of lavas. This thick zone of dense rock must be 

 reckoned with in all hypotheses that involve the penetration of air and 

 water from without, and, as well, the extrusion of lavas from within. In 

 addition to the difficulties of the penetration of ground-water, the 

 limitations of its heat, at penetrable depths, also bear adversely (see 

 p. 219), on the descent of air and water. 



Hypothesis 4. Lavas assigned to relief of pressure. — It seems to be 

 demonstrated that pressure raises the melting-point of average rock, 

 and hence at twenty or thirty miles' depth there may be rocks hot enough 

 to melt at the surface, but still solid because of high pressure. If this 

 pressure were in some way relieved they would become liquid. Pres- 

 sure may be locally reheved somewhat (1) by denudation, (2) by certain 

 phases of faulting, (3) by anticlinal arches, and (4) by continental 

 deformation. 



(1) In most cases of denudation, cooling below probably keeps pace 

 with loss above. At any rate, many volcanoes rise from the bottoms 

 of the oceans where no denudation takes place, and this phase of the 

 hypothesis is not workable there. 



(2) The theory of rehef by faulting finds encouragement in the fact 

 that many volcanoes occur on fault-lines. There is no evidence, how- 

 ever, that this is a universal or necessary relation. Computation as to 

 the amount of lowering of the melting-point that might arise from the 

 faulting associated with volcanoes indicates that it is necessary to sup- 

 pose that the rocks were already very close to the melting-point when 

 the faulting took place, to make the doctrine applicable. It is to be 

 observed that in faulting the relief of pressure on one side of the fault- 

 line is likely to be balanced by increased pressure on the other side, and 

 that this difference in pressure may be lost by distribution at a depth of 

 20 or 30 miles, where, at the nearest, this delicate relation between 

 solidity and liquidity, on which the theory is dependent, may perhaps 

 be reached. 



