OAHU.
The geology of Oahu has been set forth quite fully in two papers by the author in the Bulletin of the Geological Society of America, 1900 and 1906, The Geology of Oahu, and Diamond Head. The annexed photograph of the relief map, Plate 6, shows admirably the general features of the topography, two parallel ranges of mountains separated by an intervening valley and both the elevated lines considerably eroded by streams of running water. The range on the southwest side is called Kaala or Waia nae, culminating at 4,030 feet, with a dozen separate peaks, and the deepest part is near the middle, over which a road has been built reaching the altitude of 1,590 feet. On the ocean side there are five prominent ridges dividing the general slope into six valleys, the largest of which, holding the village of Waia nae, measures six miles from the crest to the sea and is about three and one-half miles wide. Each valley has been excavated by running streams, and the erosion has been greatest upon the slope facing the water, though there may he others of equally large dimensions now concealed by later flows It is evident that the Kaala area represents the original island : igneous dis charges produced a dome bordered by marine strata and traversed by subsequent injections. Later the copious rains brought both by the trade wind and the Kona storms channeled out deep val leys upon both sides. This island existed for many ages before another larger volcanic mass was developed in the Koolau range, and when the uppermost igneous sheets were in motion, the cor rugated eastern border of Kaala was covered by the advancing lavas. This fact was first observed by Professor J. D. Dana and is illustrated by a photograph, Plate 7, by Roger Sprague. Kaala is easily recognized by its greater height, and the plain consists of the later basalts that flowed westerly from Koolau. The view is taken from near the summit of the lowland, or the divide be tween Pearl City and Waialua, at Wahiawa. The plain consists of soft material rendered plastic by the decay of the originally hard basalts. The structure is obvious when one examines the sides of the canyons.
The Koolau range is divided into two parts, the more northern Koolauloa fifteen miles long, and the more southern Koolaupoko twenty-two miles long. The most pronounced ravines on the
west side are upon Koolaupoko. At first, because of the lack of information, it was supposed that ravines were rare in the north ern section, but Plate 6 shows that valleys are well developed all along the western slope. The highest of the Koolauloa peaks reaches 2,360 feet.
Koolaupoko has several peaks that are higher. Beginning at the north end is one not named, 2,80o feet, at the head of Halawa valley. Lanihuli on the north side of the only road crossing the range at the Pali is 2,778 feet, and Konahuanui on the south side is 3,108 feet. Farther southeast the crest of the mountains runs more easterly, terminating in a cliff six hundred and forty-two feet high at Makapuu point. The eastern slope has been greatly eroded by the rains connected with the trade winds. Two sub ordinate ridges enclose Kaneohe Bay, the more northern, Kualoa, being opposite the meeting of Koolauloa and Koolaupoko, and the more southern extending from Konahuanui to Kaneohe point. The greater size of this valley seems to be due to a concentration of the erosive agencies, seen also in the excavation of the wind gap pali 5,207 feet, and the nearest approach to it of a second gap at Kalihi perhaps 200 feet higher. A third gap is in the Koukonahua gulch leading up from Wahiawa. Olamano is an isolated peak, needle-shaped, 1,693 feet, practically inaccessible. It is a relic of the former general slope to the sea from Kona huanui. At the Pali is a cliff about a thousand feet high, cele brated in history as the scene of a catastrophe, when a victorious army forced its adversary to fall over the steep slope and lose their lives. The word pali is Hawaiian for a precipice.
When viewed from the east the precipice at the Pali is seen to be corrugated like the ribs of a domestic washboard. Plate 8 rep resents a corresponding cliff on the west side of Kaala. The visible part of the cliff must be about 2,000 feet high, and at its base is situated the plantation of the Makala Coffee Company. Like the related steep escarpement of the Arizona province the recession is precipitous, but here there are added numerous val leys rendering the whole surface corrugated.
The deeply eroded flanks of Koolauloa and Koolaupoko finely illustrate the modern doctrines of subaerial erosion.