The Santa Barbara Islands, clearly visible from the mainland on a clear day, have been the source of much speculation by geologists regarding the type of rocks and structure occurring on them. In fact, less is known about these islands than almost any other part of southern California. In an early report of the California State Mining Bureau2 a few remarks were made concerning the rocks on some of the islands. Later, A. C. Lawson3 noted the physiography and rocks of San Clemente and Catalina Islands, and W. S. T. Smith4 published a paper on the physiography of the islands which, even though the various formations were not described, is the most complete account that they have received. Since these islands have been commonly considered to be largely volcanic in origin, it was with surprise that the writer found that Santa Rosa Island is composed mainly of sedimentary rocks . . .