Differences in remanent magnetization directions from oriented samples, after partial ac demagnetization, offers a means of differentiating mafic igneous rocks of (1) late Precambrian (1400 to 1300 m.y.), (2) early Paleozoic (600 to 450 m.y.), and (3) Late Cretaceous to Holocene (<75 m.y.) age in the eastern Rocky Mountains of Colorado. Remanent magnetization directions of late Precambrian rocks have northeast declinations and negative inclinations or southwest declinations and positive inclinations; lower Paleozoic rocks have southeast declinations and either low-angle positive or negative (generally positive) inclinations; Late Cretaceous to Holocene rocks have essentially north declinations and positive inclinations or essentially south declinations and negative inclinations.