If two bodies with different temperature are in contact with one another, the temperatures will gradually equalize themselves. Fourier’s general differential equation covers all possible variables affecting the thermal history of the two masses. Such an equation, however, is not directly useable, and particular solutions must be sought which will encompass the conditions in a given problem. In such a solution, it is generally necessary to make simplifying assumptions in order to treat the problem mathematically. The resulting particular solution is commonly only an approximation, though such an approximation may be very close to actual conditions. The fewer the simplifications introduced in any problem, the more accurate will be the answer, but the more complex the treatment becomes. In applying the theory of heat conduction to geologic problems, interest is in the general picture rather than in the minutiæ; but, though a rough sketch is far better than . . .