•J 1 c. II. III'J'CI I COCK — GEOLOGY OF OAIII 



mately connected with it, which seems to have been formed in a different 

 way and in later periods. 



On tiie leeward side of Koolaupoko notice has already been taken of 

 about 20 canyons in as many miles. This is where the island is narrow 

 and the rainfall is ample for the work accomplished, though the erosion 

 lias been less than on the windward side. Relatively little work has 

 been done farther to the northwest, all the way to Waialuaand 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 

 proceeded far enough to excavate gorges high up, nor amphitheaters. 

 The shallow canyons on the north shore and in Ewa are certainly sug- 

 gestive of a very scant or recent action.* From any hill like Punchbowl 

 or Leilono one can see a fine long stretch of this sloping 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 great excavations have been effected on the west side, 

 as evidenced by the valleys of Waianae, Makaha, etcetera, while the 

 gradual slopes of the Koolau area impinge closely on the latter, and the 

 drainage has been forced westerly. The work accomplished has been on 

 the southwesterly side, whereas the trade winds have blown from the 

 northeast for nine months of the year. Shall we say either that there 

 must have been a greater fall on Kaala in ancient times, or that the 

 present precipitation of moisture has been adequate for the results ? 

 Such views are common, and had been expressed in the first draft of this 

 paper. It was recalled that the wind blew from the west on our own 

 visits to that region ; also that the erosion is effected more by sudden 

 downpours than by ordinary rains, and that consequent^ the existing 

 fall is sufficiently adequate. Opposed to this is the general aridity of 

 the Waianae region as contrasted with the abundant verdure of Kaneohe 

 bay. Reflection has suggested a better view. The Kaala dome existed 

 before the Koolau mountains were raised very much above sealevel. 

 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, and then carved out the valleys on the 

 leeward side, together with the Waianae wind gap. Two or more lengthy 

 ridges have been mentioned as protruding easterly from Kaala. 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 

 there is the plateau with gentle dip covering the interior of the island, 



*On both sides of Ewa the slopes arc protected from heavy rainfall by the opposite ridges ; bu( 

 from s.iit lake to Koko the Koolau mountain is exposed to the kona <>r southwest storms : hence 

 the deeper canyons. Note by S. E. Bishop. 



