﻿J. D. Dana — Dissected volcanic Mountain. 251 



at M on the map) ; and my return was by the west side of the 

 Papaua Valley, that next west of the Matavai Valley. The 

 top of Aorai was found to be " not six feet broad ; beyond it, 

 the mountain ridge continued for a short distance, with the 

 same sharp, knife-edged character, and then was broken off by 

 the Punaavia valley," while "at its western foot appeared 'The 

 Crown.' " Across "a valley, 3000 to 4000 feet deep, stood Oro- 

 hena, only two miles distant, with nearly erect sides and 

 equally sharp crest; and similar features characterized the other 

 ridges in the panorama. 



I refer the reader to my Expedition report for the details, 

 and for a discussion on valley-making about volcanic mountains 

 and elsewhere, including the formation of the amphitheatres 

 and a detailed description of the steps in the progressing ero- 

 sion changing the finished volcanic cone into a Tahiti, illus- 

 trated by facts from Hawaii and elsewhere. " Mount Loa, 

 alone, contains within itself the material from which an island 

 like Tahiti might be modeled that should have nearly twice 

 its height and four times the geographical extent." (Rep., p. 

 392).* 



II. The inner Structure of a great Volcanic Mountain. 



The above facts with regard to the dissected island have 

 their chief interest as an introduction to the second subject of 

 my paper — The inner structure of a great volcanic mountain. 

 The profound dissection has laid open the island to its very 

 core, and to a depth below the original summit of certainly 

 more than 4000 feet. If, as we may suppose probable, the old 

 cone had, like Mt. Loa or the Maui volcano, a great pit-crater 

 at top, the interior must be open to view for at least 3000 feet 

 below the bottom of the crater. 



Although my time at the island did not allow of a thorough 

 exploration of the central region, one point of fundamental 

 significance was made out, and put beyond question: that the 

 mass of Orohena was not composed of beds of lava. There 

 were no horizontal (or nearly horizontal) lines or divisions. 

 Instead, there were vertical lines indicating vertical joints or 

 an imperfect columnar structure, in the unstratified mass ; and 

 this was true for the mass to the very top. 



* Descriptions of the results of erosion are given in my Report, in connection 

 with the special accounts of several of the islands visited, and of Australia, and 

 in chapters on the origin of valleys, on pages 379 to 392 and 526 to 533. A brief 

 statement of the general results is made in my Manual of Geology, p. 642. 



A similar history of the process with similar descriptions of " knife-edged 

 ridges," amphitheatres, etc.. is contained in Capt. Button's Report on the Hawaian 

 volcanoes, published two years since by the IT. S. Geological Survey. As he 

 makes no mention of my volume, or even of its title or its author, in his work, I 

 may add that his readers can at least supplement their knowledge on the subjects 

 treated by consulting my report of thirty-seven years since. 



