Most of the common non-opaque accessory minerals (apatite, zircon, sphene) may be expected to crystallize late in igneous rocks since, other things being equal, constituents in small amount in a complex solution should crystallize late. This inference is confirmed by the association of these minerals with pegmatites, the presence of apatite and sphene in certain contact metamorphic deposits, and the lack of concentrations of these minerals in the case of rocks and ores produced by crystal settling at the base of complex differentiated sills and thick flows. Texture and distribution of these accessories in igneous rocks also demonstrate their late origin.The association of accessory minerals with biotite, hornblende, magnetite and, in some cases, quartz is demonstrated and attributed to their late magmatic crystallization. Conversely, the association of the accessory minerals with biotite in granites implies a late origin for this mineral even in acid rocks. The more nearly equal distribution of accessory minerals among the essential constituents of paragneisses as compared with igneous rocks offers a criterion for determining the origin of gneisses.These conclusions have a bearing on the origin of late magmatic ore deposits.