Two episodes of igneous activity occurred on the Colorado Plateau during Phanerozoic time. Laccolithic centers whose ages cluster about the Cretaceous-Tertiary boundary occur along the projection of the Laramide mineral belt of Colorado as far southwest as the Carrizo Mountains. Laccolithic and mafic volcanic centers of mid-Tertiary age occur scattered about, and on the edges of, the Plateau Province, and in some areas mafic volcanism continues today. Both pyroxene and hornblende separates consistently gave anomalously old K-Ar dates. It was thus necessary to analyze whole-rock samples from which the mafic minerals were removed to obtain age determinations that did not conflict with stratigraphic relations.