Major- and trace- element analyses of samples from the upper and lower members of the Crescent Formation demonstrate that these volcanic rocks are dominated by tholeiitic compositions, with minor amounts of alkali basalt also present. Basalt flows from both members have undergone little differentiation, and are compositionally similar. Differences in TiO2 and REE contents demonstrate, however, that these members are geochemically distinguishable. Lower-member tholeiites are identical to ridge tholeiites and are interpreted as representing a slice of oceanic upper crust. The upper member is similar to oceanic island basalts, but its primary tectonic significance is uncertain. It is postulated that the lower member was generated at a mid-ocean ridge and was subsequently subducted to a shallow level. Miocene uplift then juxtaposed the upper and lower members.