V10 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



not fairly be called fissure eruption, though it might be accompanied by lava 

 floods emitted from fractures in the roof-rock surrounding the foundered area. 

 The level of the lava in the area of foundering would, in form, resemble a 

 plateau (fissure) eruption, but the lava would here be generally liparitic rather 

 than basaltic as in the majority of plateau eruptions. Moreover, the liparite 

 would form a continuous mass, merging downwards into granite, and thus 

 not a series of superposed distinct flows. According to the topography, the 

 lava of the foundered area might flood valleys outside that area. If the 

 hydrostatic adjustment were accomplished in stages, successive superposed flows 

 might be caused in the valleys. 



Though the field evidences do not seem to favour this conception for most 

 exposed batholiths, it should be retained as a possibility in some cases. In 

 general the problem has a peculiar difficulty. The evidence for local foundering 

 is in special danger of being obliterated. The glassy or scoriaceous phase of the 

 * batholith ' will necessarily be eroded away before the granitic phase can be 

 exposed. The liparitic phase need extend to a depth of no more than a few 

 hundred feet, where it would rapidly merge into the holocrystalline phase. 

 Therefore, comparatively little time would be required to remove the original 

 surface phase. The geologist, studying the erosion-surface, might have no 

 inkling that the ' batholith ' had not been completely covered by a roof of 

 country-rock. The former existence of a roof cannot be assumed simply because 

 a ' batholith ' has a holocrystalline structure. 



The application of this deductive scheme of thought to actual examples 

 cannot be described in this chapter; it will be made in the special paper to be 

 published. The possibility that the unrivalled liparite plateau of the Yellow- 

 stone Park was formed in consequence of local foundering will there be discussed 

 in some detail. Other cases, in Massachusetts and elsewhere, will be used as* 

 parallels. 



Central Erwphons. — In many abyssal injections the magma does not 

 directly reach quite to the Earth's surface, though it may ultimately cause true 

 volcanic action in the form of ' central eruptions.' 



Mere hydrostatic outflow of magma will not explain the persistence of 

 activity at a central vent; at Kilauea there is no overflowing, though its lava 

 lake is probably the most persistently active on the globe. Recurrent explosion 

 allowing an intermittent rise of new, hot magma in the vent is clearly unable 

 to supply heat fast enough; Kilauea is not explosive. Actual calculation of the 

 connective gradient shows how powerless thermal convection is to supply the 

 necessary heat at the surface. Yet continued activity means victory of the 

 lava column in a struggle with cold. 



More promising is the conception that the heat of the underlying magma 

 chamber is transferred to the crater by another kind of convection, that due 

 to the generation of gas bubbles in the lava column. At the depth of a few 

 hundred feet, bubbles of individual mass corresponding to normal lava vesicles 

 must have very small volume. For that reason, as well as through the consider- 

 able increase of magmatic viscosity with pressure, such bubbles must rise very 



