these two mountainous areas, the foundation rock everywhere is basalt, disposed in layers dipping quaquaversally from the central lines. Kaala was an elliptic, Koolau an elongated dome, each with its seaward sides sharply incised by canyons, and both joined together by a later formed plateau, slop ing both northerly and southerly. Dana calls Oahu a "volcanic doublet," the united work of two great volcanoes which have been so greatly eroded that the proper position of their craters is now conjectural. This view is confirmed by a comparison with the Island of Maui, where one of the volcanic masses has suf fered but slightly from erosion and the connecting plain is nearly at the sea level. Assuming that there were originally two vol canic domes, with layers dipping outwardly some five degrees, it remains to apply the principles of geomorphy to explain their present forms and their relative ages. These principles were ad mirably set forth by Professor Dana in his report on the origin of the valleys and ridges of the Pacific They have been applied later to Oahu, more especially by Captain C. E. In the volcanic islands of the Pacific the original form of the land was that of a dome, consisting of basaltic layers of variable hardness, whether solid, vesicular, or agglomeratic, and sloping gently outward in all directions. An abundant rainfall is assured by the contact of the moist air of the trade winds with the ele vated mass of land. The resultant streams wear out canyons radiating from the centers or branching from axial lines of eleva tion. Of the two erosive forces, disintegration and transporta tion, the latter is the most effective in these volcanic layers, which appear almost like the strata of sediments. In case the rainfall is unequally distributed on the flanks of the elevation, the amount of erosion will vary, as may be seen in the number, shapes, and depths of the valleys excavated.
Because the transporting power of water is greater where the slopes are steep, the valleys become larger in their upper reaches, portions of the dividing ridges disappear and amphitheaters re suit ; outliers shape themselves out of the original plateau and at the confluence of tributaries ; the spaces between the streams nar row to knife edges or may disappear ; the walls, originally ver tical, change to slopes through the separation of blocks by gravity, which form a talus at the bases of the cliffs. Although frost is absent, so easily are the fragments separated because of the char acter of the rocks that the excavation is as effective as in colder climates on the more durable ledges. In the lower reaches the streams take winding curves, and thus act laterally against the sides, widening the bases.
The Koolau area is the easiest on Oahu to understand. From the details already presented it is seen to he elliptical, nearly forty miles long, and deeply eroded along its seaward face, with many amphitheaters, outliers, and especially the long cliff opposite Kaneohe Bay. There has been great excavation along the west ern side of Koolaupoko, but comparatively little on the interior side of Koolauloa. Judging from incomplete observations on the rainfall for the past five years, the average has been one hun dred and forty-four inches two miles below the Pali (Luakaha), and about twenty inches near the wharves of Honolulu ; but the rainfall is confessedly greater at the crest of the ridge, probably two hundred inches, and it diminishes gradually all the way to the harbor. The fall along the eastern shoreline exceeds thirty inches, increasing to the summit; hence it appears the water should be most abundant along the crest of the range, but greater on the eastern than the western slope, and whatever the fall may be on the Honolulu side it came from the northeast. The erosion has been the greatest on the northeastern side, as seen in the Pali, the outliers, sometimes 2,000 feet high, the ridges running northeasterly, and the amphitheaters. It reached probably to the central axial line of elevation opposite Kaneohe Bay. The cliff can not very well have been eroded by the sea, since there are irregular ridges and chains of hills at intervals of two or three miles stretching out perpendicularly from the wall and ending in promontories. Marine action would have removed these pro jections. The erosion seems to have been most intense at the road crossing the Pali, since there is a gap worn down to 1,207 feet from about 3,00o feet on either side, and there are two other gaps to the north not far away. Some have explained the pres ence of the Pali gap and the horseshoe form of the land from Mokapu point to Konahuanui and thence along the main range to the northeast branch, ending at Kualoa point, by assuming a break or fault at the Pali gap or the existence of an enormous crater in the part of the circular ridge just delineated. The best argument in reply to both these volcanic theories is that the topo graphy is in better agreement with what is known elsewhere to be the result of subaerial erosion. If there were one transverse fault, there must have been three, quite close together, for the first cataclysmic theory ; and the theory of the large crater as sumes that certain cinder cones and scoria were intimately con nected with it, which seems to have been formed in a different way and in later periods.