The order in which the various mineral constituents of an igneous rock may crystallize out from an igneous magma offers to the petrologist a problem of great difficulty and complexity. It is generally admitted that the order of consolidation is not wholly determined by the order of the fusion-points of the constituents, and with this admission the fusion-points have come to be consistently disregarded, as though they might safely be left out of account. That this is not the case has of late become strongly impressed upon me, especially after a consideration of the important data obtained by Mr. Ralph Cusack, B.A., who, by means of Professor Joly's meldometer, has determined the precise temperature of fusion of most of the rock-forming minerals.