484 FRANK F,. GROUT 
Daly’ in explaining the supply of heat from depth to lava lakes— 
he called it two-phase convection. 
It seems perfectly clear, from a general consideration of the 
idea, that a mass of lava filled with bubbles would have a lower 
‘“agoregate specific gravity”’ than a neighboring mass without such 
bubbles. Any process of local vesiculation would almost certainly 
result in convection. It is perhaps less clear and less often empha-. 
sized, but none the less true, that a mass of lava in which crystals 
have formed has a greater “aggregate specific gravity’ than it had 
just before, and a local development of crystals would also almost 
certainly start convection. In the case of gas bubbles in a magma 
the escape of the gas from a crater lake would finally remove the 
cause of the circulation. In order to maintain the circulation there 
must be a continuous supply of gas bubbles to some portion of the 
magma. Similarly, if crystals settle out of a liquid magma they 
would no longer tend to move the liquid. The circulation caused 
by the density of crystals would be active only during the time in 
which crystals were developing locally in the liquid and settling 
through it. However, in a large body of magma either of these 
processes might be maintained for a long time. 
THE DEVELOPMENT OF PHASES IN MAGMAS 
Phases are defined in physical chemistry as the parts of a system 
which are mechanically separable. A simple magma consists of 
one liquid phase. Considered as a solution, the magma may con- 
tain various dissolved substances, including many minerals as well 
as gases and water; but it remains one phase and only one. 
Gas phase-——When bubbles of gas separate from solution in a 
magma and remain in it as parts of a closed system (i.e., do not 
mingle with the outside air), they may be considered a second phase 
in the magma. The conditions and reasons for their separation 
are various. The solubility of gas in a liquid varies with the 
pressure. If a magma saturated with gas at a pressure of 200 
atmospheres is erupted to the surface, where the pressure is one 
atmosphere, gas will separate from the solution. It is equally sure 
=R. A. Daly, ‘‘The Nature of Volcanic Action,” Proc. Am. Acad. of Arts and Sci., 
ME VIL, 76: 
