The Columbia River basalt has been sampled by 28 stratigraphic sections and by the mapping of selected areas. Petrographically, and also stratigraphically, the rocks assigned to this formation by Merriam, Lindgren, and Smith are divisible into 2 distinct varieties. The older, widespread in the Imnaha and John Day regions, is characterized by about 5% olivine, a silica content of 47 to 50%, and by notably higher Al 2 O 3 , MgO, and CaO than the younger. Many outcrops have a characteristic "greasy" appearance because of the presence of saponite after olivine and of nontronite and other clay minerals after chlorophaeite. The basalts at Picture Gorge and Turtle Cove, described by Merriam in 1901, are typical. The younger basalts are characterized by more than 20% tachylyte, little or no olivine, a silica content of 53 to 54%, and by notably greater amounts of K 2 O and TiO 2 . Nearly all the basalts that occupy the central and eastern part of the lava field N. of the John Day and Imnaha areas are of this kind. They constitute the Yakima basalt as defined by Smith in 1901. In the lower part of the Imnaha River canyon, and also in the John Day basin, the Yakima basalt rests with distinct structural unconformity on flows of the older Picture Gorge type. A late variant of the Yakima basalt emerged after warping and faulting had started to deform the Yakima flows in early Pliocene time. It contains more olivine and plagioclase and a distinctly higher percentage of Fe and titania than normal Yakima flows. Because of their separation in time, differences in chemical and mineral composition, and particularly because of the absence of transitional varieties between them, these lithologic and stratigraphic variations of the Columbia River basalt are considered to be products from separate magmatic hearths, and not differentiates of a hypothetical uniform magma.