370 Transactions . 



So far as the actual located outcrops are concerned, the extreme types 

 appear to be marginal in their occurrence, while the main mass of the central 

 hill — Ahititera — seemed to be composed of monzonitic rocks, judging by 

 the boulders contained in the beds of the streams that radiate from the bill. 

 It was most unfortunate that the slopes of Ahititera were not more closely 

 examined, but as our route had led along a stream-valley in mountainous 

 country covered with tropical forest it was not possible to get an idea of 

 the topography of the country until the viewpoint from the pass Tetiairi 

 had been reached, and it was then too late to return. 



As stated previously, the hill Ahititera, over which the outcrop of 

 plutonic rocks occurs, is the actual central area of the island. This hill 

 is almost entirely surrounded by a circle of high mountain-peaks, composed 

 apparently of lavas and breccias, and from these peaks the land slopes 

 outward without steep average slopes in all directions. It is, however, 

 radially furrowed by deep radiating valleys separated by the knife-like 

 ridges so well described by Darwin and Dana. On the west side only is 

 there any exception to this statement. There Aorai and Diademe extend 

 the high country somewhat to the west of the upper part of the Papenoo 

 Valley. 



When this arrangement of a small plutonic area surrounded by a circle 

 of volcanic rocks, and of this central hill surrounded by a ring of lofty peaks, 

 is borne in mind, only one conclusion as to the structure of the island can 

 be arrived at. It appears obvious that the plutonic area marks the position 

 of the much-denuded remnant of the plug filling the pipe through which 

 the volcanic rocks were ejected. If this reasonable conclusion be accepted, 

 it will be seen that the original volcano of which Tahiti was formed reached 

 a height of at least 3,000 metres, for the dissected and worn remains still 

 attain an elevation of 2,232 metres in Mount Orofena, which stands some 

 distance back from what was probably the central orifice of the volcano. 

 Here, then, we have the materials of a volcanic plug exposed by erosion 

 and weathering at a depth of 2,000-2,500 metres below the summit of a 

 volcano the ruined flanks of which still rise up in a mighty rampart to a 

 height of 1,500 metres or more on every side. 



The plutonic rocks of the central plug of the volcano are mainly of 

 an alkaline nature, lying between monzonite and theralite. The volcanic 

 rocks are mainly basalts, though hauynophyres and phonolites also occur. 

 There thus appears to be something of a discordance between the plutonic 

 and volcanic types, and this even suggests that there is no community of 

 origin between them. This discordance is of a specially marked nature 

 mineral ogically, for whereas almost all the plutonic rocks contain a large 

 amount of nepheline, brown hornblende, and often biotite and sphene, the 

 volcanic rocks contain practically none of them. 



This difference is, however, far more marked mineralogically than 

 chemically, as has been well shown by Lacroix. The analyses that he 

 has published show quite clearly that the chemical differences between 

 his gabbro nephelinique (theralite) and gabbro essexitique on the one hand, 

 and of some of the basalts on the other, is quite slight. Again, the syenite 

 nephelinique a amphibole and monzonite nephelinique on the one hand are 

 also chemically equivalent to the hauynophyres on the other. The discoverv 

 related here of the plutonic rocks syenite and peridotite now provide 

 equivalents for the phonolite and picrite of Lacroix among the volcanic rocks. 



It thus appears that from a chemical standpoint there is a satisfactory 

 equivalence between the plutonic and the volcanic rocks of this island. 



