J. D. Dana — Geological History of Oahu. 91 



for further explorations, refreshing old memories and adding 

 new facts ; and this return to the subject affords an occasion 

 also for reconsidering former conclusions. 



1. Features, structure, and origin of Oahu. 



1. General features • Contrast with the island of Maui. — 

 Like Maui, Oahu is in origin a volcano-doublet — that is, as re- 

 gards rock-structure, it was the united work of two great vol- 

 canoes, a western and an eastern. But unlike Maui, its two vol- 

 canic cones or domes have suffered so great loss that the posi- 

 tion of either crater is wholly a matter of conjecture. 



A large part of the loss Oahu has suffered is due to denud- 

 ing agencies. East Maui, as the map on Plate 3 illustrates, has 

 lost in this way comparatively little of its original evenness of 

 surface owing to the recency of its extinction. Its windward 

 gorges are narrow, and only shallow gulches occur over the lee- 

 ward surface. The ratio of its diameters at base, 1 : 1 - 3, is 

 probably very near the original ratio. West Maui is pro- 

 foundly gorged on all sides and most deeply so to windward, 

 illustrating results of longer wear than East Maui has had. 

 But something of the old slopes remain, and in the base we 

 have still the ratio of its old diameters, 1 : 1"4, with the outline 

 little indented. The double lesson is taught : (1) what denud- 

 ation from descending waters does to a volcanic cone 5° to 10° 

 in slope in the region of the trades ; (2) what, on the contrary, 

 the sea cannot do, no encroachments of note existing to attest 

 to its power, notwithstanding the length of the era of denudation. 



Oahu resembles Maui in having the western mountain-cone 

 the most time-worn and the smaller in area, but here the like- 

 ness ends. Both of its mountains are deeply eroded. Further, 

 East Oahu has only part of its old slopes left. They remain 

 only on its southern, western and northern sides ; the north- 

 eastern are cut off by the great precipice, twenty miles long, 

 which is made for the most part of the edges of the lava- 

 streams that slope southward and westward. The sharp-edged 

 serrated ridge, making the summit of the precipice, is from 

 1000 to 3000 feet in height, and at its northeastern base, from 

 Kualoa eastward, there is in general only a narrow strip of low 

 land with low hills, the width but three or four miles except 

 in the Kaneohe peninsula. The precipice continues beyond 

 Kualoa northwestward, but not the low land at its base. 



These features have occasioned peculiarities in the results 

 of denudation on East Oahu. The leeward or south and 

 southwestern sides have long and deep valleys, some of them 

 heading in broad amphitheaters under the crested mountain 

 riclge. The windward side, along the 20-mile precipice, on 

 the contrary, has buttresses and shallow alcoves, with a but- 



