310 N. L. BOWEN 
proportional to the absolute temperature and that for this reason 
diffusion takes place until the concentration is inversely proportional 
to the absolute temperature. In other words, the effective con- 
centration is, initially, inversely proportional to the absolute 
temperature, and diffusion takes place until the effective concen- 
tration is uniform. In applying these considerations to a cooling 
mass of rock, we may take for simplicity a tabular body. A solu- 
tion of the problem of the cooling of such a body is given by the 
equation 
l—x 
2V kt 
Oo 
== e—*'dB 
Vo 
—l—x 
2V kt 
where @ is the temperature at any point distant « from the margin, 
6, the original temperature of the magma, the temperature of the 
wall rock being taken as zero and / is thickness of the intrusive. 
If we take a tabular body of thickness 20 m. (i.e., 10 m. from 
center to margin) we may calculate the temperature in any plane 
at given distance from the margin at the end of any period of time. 
The results of such calculations are shown graphically in Figure 6, 
the curves representing the distribution of temperature at the end 
of various periods of time if the diffusivity is taken as 0.0118 
in cm.? per second. In this figure the temperature scale has no 
necessary absolute significance, o of the scale being merely the 
initial temperature of the surrounding rock and 1 of the scale being 
the initial temperature of the magma. It will be noted that at the 
end of one year the temperature at the margin is about halfway 
between the initial temperature of the surrounding rocks and that 
of the magma, while the temperature at the center is much higher. 
If it is supposed that the whole mass is still above its crystallization 
temperature, then the Soret action should be operative, that is, 
the effective concentration at any point should be proportional to 
the absolute temperature, and diffusion should take place until the 
effective concentration was uniform. Partly for simplicity and 
partly for the sake of obtaining an especially marked Soret effect 
we shall assume that the temperature scale of Figure 6 represents 
