Introduction -- Regional geology -- Description of the units -- Olivine basalt porphyry -- Tuffaceous shale and volcanic sandstone -- Lower submarine breccia -- Palagonitic pillow breccia -- Crystal vitric tuff -- Interlayered basalt flows and flow breccias -- Upper submarine breccia -- Basalt flows capping Cedar Butte -- Basaltic dikes -- Composition of the basalts -- Structural geology -- Summary and conclusions -- Acknowledgments -- References cited.