DALY, 



THE NATURE OF VOLCANIC ACTION. 



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contain, on the average, at least 200 vesicles per cubic centimeter of 

 the lava. The vesicles of the surface layers are roughly spherical and 

 average no more than 2 mm. in diameter, though, of course, the range 

 of diameters is very great. For convenience, let a spherical mass of 

 hydrogen, having the radius of 1 mm. at one atmosphere of pressure 

 and at 1200° C, be called the "standard bubble " for basalt. Extra- 

 polating on Amagat's pressure-volume curves for hydrogen at 1200'^C., 



S/acA 



FiOTRE 6. Diagrammatic section of Halemaumau, illustrating two-phase 

 convection, erosion of caves, and vortical action. The lava " scum " is ropre- 

 senterl by the heavy black line at the lake surface. Length of section about 

 300 meters. Vertical scale specially exaggerated in drawing the " scum " 

 line. 



the volumes and radii for the standard bubble at high pressures may 

 be calculated within a margin of error which is probably very small. 

 Examples are shown in the following table : 



Gas-free ba.salt at r200°C. and one atmosphere has a specific gravity 

 of about 2.75. On account of the slight compressibility of rock-matter, 

 that value may be assumed as typical for gas-free basaltic magma at 

 I)res.snres up to several thousand atmos])heres. If such magma become 

 charged with 200 standard bubbles per cubic centimeter, at 200, 

 loon, and 2000 atmospheres, the specific gravity falls to the following 

 approximate values (Col. 1) : 



