﻿THE 



AMERICAN JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



Akt. XXVIII. — A dissected volcanic Mountain ; some of its reve- 

 lations ; by James D. Dana. 



The island of Tahiti — the largest of the Society Islands — 

 affords excellent illustrations of two widely different subjects : 



I. Erosion, by running waters ; and 



II. The inner structure of a great volcanic mountain. 



I have described the features of the island in my Geological 

 Report of the Wilkes Exploring Expedition,* from observa- 

 tions made in 1839, and have pointed out the bearings of the 

 facts on the two subjects, but I give there no map of the topog- 

 raphy, or views of the peaks ; and I now return to the sub- 

 ject to add these illustrations with some further remarks. 



The island has nearly the shape, as regards outline, of a let- 

 ter 8, and was once a twin of volcanoes. Only the northern 

 and larger of the two peninsulas is often visited, and of that I 

 speak. It was originally a gently sloping cone of the type 

 represented by the Hawaian volcanoes ; for its beds of lavas, 

 as seen in the sides of the valleys, slope at a small angle 

 toward the shores ; mostly 3° to 10° — varying in some parts to 

 15° — on the north and west sides, where my examinations were 

 made. Supposing the mean slope to be 8° the height above 

 the sea-level of the original cone — the diameter of the island 

 being twenty miles — would have been nearly 7500 feet. But 



* Pages 283 and 386 of the quarto volume (of T56 pp. with a folio Atlas) pub- 

 lished in '1849. See also Abstract of part of the results, this Journ., II, ix, 50, 

 1850. 



Am. Jour. Sci.— Third Series, Vol. XXXII, No. 190.— October, 1886. 

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