102 
IDDINGS. 
approaches the surface. At some particular moment the 
ratio between them may be such that the rate of loss of the 
heat exceeds that of the decrease of pressure, and the magma 
reaches a degree of saturation that necessitates the crystal¬ 
lization of certain of the compounds in solution. As long as 
the changes in the heat and pressure continue in the same 
direction the crystallization of the magma will progress. 
When the magma reaches the end of its eruption the 
pressure will remain very nearly constant, while the heat 
will continue to decrease, but at a different rate, which will 
depend on the environment of the magma. It may decrease 
more slowly or more rapidly, and with it the character of 
the crystallization will vary. 
When a column of magma has come to rest and fills its 
conduit the pressure at various places in the column will 
remain nearly constant, but will be greater from point to 
point downward. The temperature will continue to decrease 
by conduction through the walls of the conduit, but from 
what has already been said it is probable that the tempera¬ 
ture of the conduit’s walls will be greater downward, and the 
actual temperature of the magma will also be greater down¬ 
ward. The rate of the loss of heat by conduction will prob¬ 
ably decrease as the distance below the surface of the earth 
increases; hence the magma should cool much more slowly 
at great depths than near the surface; but the effect of the 
increased pressure upon deeper and deeper portions of the 
magma would be to increase the degree of saturation for any 
given temperature or to raise the point of temperature at 
which supersaturation and crystallization sets in. It would 
consequently tend to counteract the effect of the increase of 
temperature in the deeper portions of the magma and the 
slower rate of cooling, and would produce greater uniformity 
in the crystallization which takes place at various points in 
a vertical column of magma. 
From the study of volcanic phenomena we know that the 
act of eruption is not always a uniform movement of a 
magma from great depths to its final place of rest; that its 
