The Koolau volcanic dome forms the eastern and larger part of the island of Oahu. It rises to somewhat over 3000 feet above present sea level and is composed almost entirely of thin basaltic lava flows. The visible flows were erupted from vents along a fissure system more than 30 miles in length and trending parallel to the main axis of the Hawaiian archipelago. The fissure system is marked by swarms of subparallel dikes in a band 2 to 5 miles wide. The rock formation marked by the dikes has been called a dike complex. In the southern half of the dome the crest and fissure portion has been cut away by erosion from the east, but the forms and structures which remain indicate that the dome at present sea level was never more than about 20 miles wide and that its form was greatly elongate, consistent with its growth about a linear vent system. Indications that faulting took a significant part in the reduction of the southern part of the windward slope are suggestive, but no direct proof has yet been found.
The remainder of the dome has been profoundly dissected to the stage of mid-maturity though enough remains to everywhere indicate its general original form. Following the period of most pronounced erosion there was a series of late eruptions both around the margins and in the interior of the southeastern part of the dome, giving rise to lava flows and pyroclastic cones of ultrabasic composition named the Honolulu series as described by Winchell (1946). Presence of these late rocks in various forms, distributed about Honolulu has attracted petrographic attention to the neglect of the main Koolau series. Following extensive field study and cutting of many hundreds of thin sections in connnection with water-supply studies, nine specimens have been analyzed with particular reference to the problem of variation. No indication of significant or systematic variation in chemical composition has been found, despite the collection of samples from widely separated points and from various depths in the dome. The Koolau basalts are slightly more silicic than most of the “normal Hawaiian basalt” averages heretofore known. Attempts have been made to derive the Koolau composition on various hypotheses, but none having independent basis of plausibility has been developed.