Relation of the Yakima Basalt to the Keechelus Andesitic Series

The Yakima basalt of eastern Washington is associated toward the west in the Cascade Mountains with the Keechelus andesitic series. The latter series has generally been regarded as the younger. In the Mount Aix quadrangle the upper part of the Keechelus andesitic series is mapped as a separate unit-the Fifes Peak andesite. Marginal flows of the Yakima basalt overlap the Fifes Peak andesite and are clearly younger than the andesite. The Yakima basalt is generally regarded as Middle Miocene but may be Upper Miocene. The Fifes Peak andesite is presumably not younger than Middle Miocene. The lower part of the Keechelus andesitic series may be no younger than Oligocene. The Snoqualmie granodiorite is probably intermediate in age between the lower part of the Keechelus andesitic series and the Fifes Peak andesite. The Cascade Mountains of southern Washington owe their present height principally to uplift rather than to the accumulation of volcanic materials.