A small tar-impregnated dyke cuts Miocene shale just east of the old Newport Beach field, Orange County, California. Heavy mineral analyses show that the material of the dyke corresponds with underlying Miocene sand rather than with overlying younger formations. The Miocene in the area is characterized by a lack of green hornblende and an exceptionally large amount of glaucophane. The reverse is true of the Pleistocene. An earthquake probably formed a fracture into which was forced water-soaked sand from a lower formation. The oil and tar which occur in the dyke and the overlying Pleistocene beds probably entered subsequently to the sand-filling of the dyke.