DALY. — THE NATURE OF VOLCANIC ACTION. 81 



corresponding density and of radii of 10 or more meters would rise at 

 the rate of at least 10 centimeters per second or 3G0 meters per hour. 



This analogy of solid spheres seems to afford some help in our 

 imagining the course of a specially vesiculated mass of lic^uid magma. 

 The rough quantitative estimate just made for large solid spheres can- 

 not be directly applied to this case. On account of the possibility of 

 internal movements in the rising mass of liquid magma, its speed of 

 uprise will not be quite the same as that of a solid mass of the same 

 shape, size, and density. Yet the correction to be applied is probably 

 small. 



As such a mass approaches the surface, through a column of rapidly 

 decreasing viscosity and with a constant increase of buoyancy because of 

 expansion of the contained bubbles, the velocity must greatly increase. 

 However much a given mass of magma might lose buoyancy through 

 the loss of its larger, more swiftly rising bubbles, the total effect must 

 be to generate a powerful upward current in the magmatic column. 



In spite of the lack uf the necessary, full experimental data, our 

 general conclusion seems to be as follows. Experiment does show that 

 the rise of individual gas bubbles in magma will be very slow. ?seither 

 experiment nor theory can as yet declare the actual speed of the rise 

 of a ma.ss of specially vesiculated magma, but the analogy of solid 

 spheres moving under gravity in a liquid enforces the belief that the 

 more buoyant magma will move rapidly if its volume is of the order of 

 thou.sands of cubic meters. A.ssuming such differential vesiculation in 

 great depth, and assuming also a mechanism by which the gas of risen 

 magma is dissipated (as in a volcanic vent), two-phase convection 

 must stir the magma column to great depth and with considerable 

 rapidity. Such a process must be incomj)arably more rapid than that 

 of thermal convection under volcanic conditions. The transfer of heat 

 may readily be conceived as able to supply the radiation loss in the 

 crater for long periods of time. 



The basal assumption, that vesiculation occurs at great depth in a 

 volcanic conduit, is necessarily ditlicult to test by the facts of liold geol- 

 ogy. During its solidification an intrusive body is likely to be cleansed of 

 its bubbles, "which ri.se, and the gas so collected at the roof is .slowly di.s- 

 sipated into the country-rock. This may be the explanation of the lack 

 of vesiculation in most dikes, sheets, laccoliths, and V)atholiths. In gen- 

 eral, the rock of a lava neck maybe similarly freed from bublilcs during 

 the relatively long period of crystallization. Nevertheless, ca.ses are 

 not wanting where bubbles are known to have been tra])])i'd in basalt at 

 depths greater than 300 meters. The basalt of the West Maui neck, 

 illustrated in Figure 10, is charged with many minute vesicles at a 



VOL. XLVII. — 6 



