The age of igneous rocks is determinable by a method based on the included accessory minerals. In the common igneous rocks, most of the lead is concentrated in the potassium minerals, and most of the radioactivity is in the zircon and other accessory minerals. The lead in the potassium minerals is believed to be mostly primary lead; that in zircon is probably chiefly radiogenic lead. By separating the zircon of fresh igneous rocks, determining the amount of lead with the spectrograph and the radioactivity by alpha counters, the age of Paleozoic and Precambrian rocks can be determined with an accuracy of approximately 90 per cent. Basalts and gabbros may contain too few accessory minerals for satisfactory age determination by this method. Zircon is the most satisfactory mineral for this kind of determination. Apatite and sphene give high results and therefore must contain primary lead. Sphene gives erratic results.