﻿248 J. D. Dana — Dissected volcanic Mountain. 



the slopes above may have been steeper and the height much 

 greater. The greatest height at the present time according to- 

 an imperfect measurement made by Lieut. W. M. Walker, 

 U. S. N. (who took as a base a line measured on the coral reef 

 near Matavai) is about 7000 feet. 



The old cone is now a dissected mountain. Valleys cut pro- 

 foundly into its sides, and lay bare the center to a depth of 

 from 2000 to nearly 4000 feet (by estimate) below the existing 

 summit; and the deep valleys so crowd on one another that 

 the dissection is complete. 



The topographic features of the island are shown on the 

 accompanying map. This map is a copy in the main of that 

 in Captain Wilkes's Narrative of the expedition. " In the 

 main," I say, because I have made changes, removing some of 

 the imperfections introduced by the art of the map-maker or 

 engraver and his want of knowledge of the region. I should 

 not have taken this liberty were it not that part of the map 

 was originally from a sketch of my own, communicated to the 

 Hydrographic department of the Expedition. My sketch com- 

 prised the northern third of the island, from the center out- 

 ward, between the Papenoo and Punaavia valleys,* and was 

 prepared from personal observations in the valleys of the region, 

 and on an ascent of Mt. Aorai, one of the two highest peaks. 

 Being the only person of the Expedition that made the ascent^ 

 no other one had the opportunity for so comprehensive a view 

 of the ridges and valleys of that part of the island.* Of the 

 central peaks, the highest, at a on the map, made 7000 feet in 

 height by Lieut. Walker's measurement, is called Orohena ; 

 the next highest, about 500 feet lower, at b, is the one called 

 Aorai. f 



I. Erosion. 



As the map shows, the island in its present condition is an 

 admirable model of a deeply denuded or water-sculptured 

 mountain cone. To appreciate the precise conditions under 

 which the denudation went forward it has to be borne in mind 

 that the waters from the rains and clouds are most abundant 

 about the summits and higher portion of the island, and are 

 there perpetually at work. In these upper parts, therefore, or 

 above 1500 feet, forests and shrubbery cover the ridges and 

 valleys wherever there is a foothold; but toward the coast, or 



* On the ascent and at the summit I took some bearings, but Tmade no attempt 

 at a trigonometrical survey, and the map is still unsatisfactory in its minor details; 

 but it well illustrates the points in view. 



On the excursion I had with me only two Tahitians. The ascent was made 

 after Captain Wilkes bad left Tahiti with his vessel, the Yincennes, and hence 

 the mistaken statament in his Narrative that I was accompanied by others of the- 

 expedition. 



f These words are pronounced O-ro-he-nah and Ow-ry. 



