TWO-PHASE CONVECTION IN IGNEOUS MAGMAS 495 
the stage of crystallization. If the cooling is rapid no convection 
of importance occurs. If the borders are chilled, convection can 
be active only in the central portions. In considering convection 
effects then, we eliminate all suddenly chilled phases. There 
remain many igneous masses of large size in which the process of 
crystallization extends over a long period." 
Where cooling occurs.—At first intrusion cooling progresses from 
all sides, but, as shown 
in the discussion of the 
temperature gradient, 
later cooling would be 
largely from the top of 
the mass and related to 
surface radiation and 
ground-water circula- Liquid Magma 
tion. However, in a 
mass of laccolithic or 
batholithic form enough 
cooling would occur at See see 
the sides to establish aE tat a Interlocking 
normally some circula- eee 
tion. Once the current 
is established it tends to 
develop in power, the Fic. 1 
cooling top layer being 
drawn over to the sides as the side layers sink. 
The column of liquid which is effective in the motion is shown 
in Fig. 1, representing the zone of cooling near the side of the 
magma. The active force is much greater than the resisting 
viscosity up to the point where crystals begin to touch each other. 
It is greatest in a zone near the solidified wall. As crystallization 
progresses all the zones move inward. The width of these zones is 
wholly uncertain, but remembering the wide temperature range 
through which crystals may be in equilibrium with a magma, it 
seems probable that 20 meters (used in calculation) is a small 
estimate for large magma chambers. 
Density D ifferences 
tending to produce convection 
Solid Rock 
Viscosity 
™R. A. Daly, ‘‘Mechanics of Igneous Intrusion,” Am. Jour. Sci., XXVI (1908), 25. 
