22 C. H. HITCHCOCK — GEOLOGY OF OAHU 



discharging into the Pearl River lagoon, and also into the ocean at Waia- 



lua (see plate 1). 



Geomorphy 



Within the two mountainous areas now outlined, 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, sloping 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 suffered 

 but slightly from erosion and the connecting plain is nearly at the sea- 

 level. Assuming that there were originally two volcanic domes, with 

 layers dipping outwardly from 5 to 10 degrees, it remains to apply the 

 principles of geomorphy to explain their present forms and their relative 

 ages. These principles were admirably set forth by Professor Dana in 

 his report on the origin of the valle} 7 s and ridges of the Pacific islands.* 

 They have been applied later to Oahu, more especially by Captain C. E. 

 Dutton. t 



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 elevated mass of land. The re- 

 sultant streams wear out canyons radiating from the centers or branch- 

 ing from axial lines of elevation. Of the two erosive forces, disintegra- 

 tion and transportation, 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 di- 

 viding ridges disappear and amphitheaters result; outliers shape them- 

 selves out of the original plateau and at the confluence of tributaries ; 

 the spaces between the streams narrow to knife edges or may disappear ; 

 the walls, originally vertical, change to slopes through the separation 



* U. S. Exploring Expedition, Geology, pp. 379-302. 

 f Fourth Ann. Report U. S. Geological Survey. 



