﻿254 J. D. Dana — Dissected volcanic Mountain. 



It can hence hardly be doubted that a massive central struct- 

 ure, like that revealed in the dissection of Tahiti, is a common 

 feature of the greater volcanic mountains that have become 

 extinct. 



The slow cooling, extremely slow, on account of the magni- 

 tude of the mass, and under great pressure moreover, would 

 have caused the lava to solidify into a compact crystalline 

 rock, and often into a coarsely crystalline rock. 



I found evidence of the massive condition and slow cooling 

 of the rocks of the interior of the island in one of the valleys 

 leading to the center. Up the Papenoo Valley six to eight 

 miles from the coast, along the stream, I found many worn masses 

 of a whitish granite-like dioryte.* It had evidently been 

 brought down the valley from one of the central peaks at its 

 head, either Orohena or an associate ; and the conclusion 

 seemed to be unavoidable that the difference between it and 

 the lavas below in compactness and grade of crystallization 

 was due to difference in pressure and rate of cooling; and I 

 say (Eeport, p. 377) that although the liquid rock of volcanic 

 outflows generally cools without distinctly visible crystalliza- 

 tion, or with only crystals of feldspar, or of augite, in a com- 

 pact base, "the cooling is sometimes sufficiently gradual to 

 allow of the whole crystallizing : and in this case, the texture 

 throughout is crystalline and the rock much resembles a gran- 

 ite," and I add further : " Under the same circumstances (or 

 even a less gradual cooling) the elements of aui'ite present will 

 crystallize as hornblende, for these two minerals are identical 

 except in crystallization, and this difference depends on temper- 

 ature and rate of cooling, hornblende requiring the slower 

 rate." In this way I explain the formation of the dioryte, a 

 hornblende rock. I thence remark (p 378) : "Thus we arrive 

 at the same statement with which we commenced, that particu- 

 lar rocks have no necessary relation to time on our globe, ex- 

 cept so far as time is connected with a difference in the earth's 

 temperature or climate, and also in oceanic or atmospheric 

 pressure ; for if the elements are at hand, it requires only dif- 

 ferent circumstances as regards pressure, heat, and slowness of 

 cooling, to form any igneous rock the world contains."! 



This conclusion had not a full basis of facts ; for I had not 

 collected specimens from the bases of those central peaks. But 

 it was right, as I have believed ever since the visit to Tahiti and 



* The rock is called syenyte in ray Eeport ; but I describe the feldspar as near 

 albite. recognizing thus its trielinic character. 1 give the specific gravity as 2'73. 

 My excursion up the valley was cut short by a circumstance beyond my control 

 and hence the specimens were not traced to their source. The specimens I left 

 in 1844 in Washington, where they were deposited under Government orders. 



f The same chapter in my Report (p. 375) contains a suggestion as to the in- 

 crease in the feldspathic character of the lavas in the central conduit of a volcano 

 through a prolonged liquation-process, which has not been sustained by facts. 



