THE EXTRUSIVE PROCESSES. 625 



and hence cannot well be supposed to connect with the same reservoir, 

 but they are both on the same vast cone, which implies at least an equally 

 large molten reservoir as its source. If there were two distinct reservoirs 

 of the required magnitude, they must be singularly placed to supply 

 vents so near and yet so independent. The difficulty grows greater 

 when the whole Hawaiian chain is considered, for the points of eruption 

 seem to have migrated from the northwesterly islands, where the vol- 

 canoes are old, to the southeastern end, where volcanic activity is now 

 in progress. 



It would be natural under this view to suppose that these residuary 

 lakelets of liquid rock should be gradually exhausted as time goes on, 

 and that vulcanism should be a declining phenomenon. It is not clear 

 that this is the case. The great number of existing volcanoes in regions 

 where great extrusions took place in earlier ages does not seem to be in 

 harmony with the hypothesis. 



II. On the Assumption that the Lavas are Secondary. 



The serious difficulties that arise in interpreting volcanic lavas as 

 remnants of an original molten mass, and the strong arguments of 

 recent years for a very solid earth, have turned inquiry chiefly toward 

 the second class of hypotheses, which refer the origin of lavas to the 

 local melting of deep-seated rock. These differ widely among them- 

 selves. One group seeks for a cause of the melting in the penetration 

 of surface air and water; another, in the relief of pressure; a third, in 

 crushing and shearing ; a fourth, in the depression of sediments into the 

 heated interior zone ; and a fifth, in the outward flow of deep-seated heat. 



Hypothesis 3. Lavas assigned to the reaction of water and air pene- 

 trating to hot rocks. — As steam is one of the great factors in the explo- 

 sions of volcanoes, and as water reduces the melting-point of rocks, it 

 is a natural and simple view that water penetrating through the fissures 

 and pores of the outer crust and coming into contact with the heated 

 rocks below, is absorbed into them and renders them liquid, and that 

 then, being rendered swollen and lighter by the process, they ascend 

 and discharge quietly or explosively according to the special conditions 

 of the case. Naturally the suggestion arises that the waters would be 

 converted into steam long before they could reach rock hot enough to 

 be melted, and that this steam would be forced back along its own track, 

 as the line of least resistance, rather than force itself into the rock mate- 



