On the leeward side of Koolaupoko notice has already been taken of about twenty canyons in as many miles. This is where the island is narrow and the rainfall is ample for the work accom plished, though the erosion has been less than on the windward side. Relatively little work has been done farther to the north west, all the way to Waialua and Waimea. A part of this lack of erosion may be due to a smaller rainfall, stated to have found its maximum at the Pali gap. Certainly erosion has not pro ceeded for enough to excavate gorges high up, nor amphitheaters. The shallow canyons on the north shore and in Ewa are certainly suggestive of a very scant or recent action. From any hill like Punchbowl or Leilono one can see a fine long stretch of this slop ing plateau, which has been utilized for the growth of sugar cane.
The Kaala dome presents phenomena of erosion very similar to those of Koolau, but the greater excavations have been effected on the west side, as evidenced by the valleys of Waianae, Ma kaha, etc., while the gradual slopes of the Koolau area impinge closely on the latter, and the later drainage has been forced westerly. The work accomplished has been on all sides, whereas the trade winds now blow from the northeast for nine months of the year. The Kaala dome existed before the Koolau mountains were raised very much above sea level. The ocean came perhaps half way across the island, and the trade winds impinged against the basaltic piles, dropping moisture, which excavated the eastern side very completely, together with the Waianae wind gap. Two
or more lengthy ridges have been mentioned as protruding east erly from Kaalaa. In later times Koolau came up from the depths and poured over the skeleton ridges on the east side of Kaala, so as to conceal them from view, and underlaid the plateau with nearly horizontal sheets of basalt. This view does not compel us to believe in the existence of climatic conditions different from those now prevailing, and it enables us to interpret what has hap pened from the varied topography. The greater excavations on the Waianae side have been effected by the Kona storms, both early and late.
This theory is confirmed by observing a more decided contrast on the adjacent double island of Maui. The smaller, older mass of Eeka, in West Maui, has suffered much greater erosion than Kaala, and has also its wind gap, while the gigantic Haleakala, which has poured out sheets of lava almost in historic times, pre sents only the modern type of canyon erosion on its wind ward side, and the leeward side has not been affected. The con trast between the two parts of Maui is more marked than upon Oahu, but it is the same in kind and may illustrate the similar sequence of Kaala and Koolau.