Rhythmic layering in the Freetown complex [Sierra Leone] and in some other layered intrusions may be due to the different powers of nucleation of the primary minerals from an undercooled magma. It is suggested that, in slowly cooled intrusions, the total number of crystals per cm 3 in a rock (whole rock index) is a measure of the amount of undercooling; rocks with a low whole rock index crystallized at or near the equilibrium temperature while rocks with a high index crystallized under conditions of undercooling. If this is so, then it is possible to determine the order of abundant nucleation of the primary minerals by graphical means. Only a slight amount of undercooling, perhaps of the order of 4 or 5 degrees C, is necessary to produce marked changes in the order of separation of the minerals from the magma.

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