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PACIFIC EXPLORATION: J. P. IDDINGS 
on the main island of Tahiti, though they are common on the lesser 
volcano of Taiarapu which is connected with Tahiti by a narrow isthmus. 
The preponderating lavas of the Taiarapu volcano are basalts like 
those of Tahiti proper. The smaller volcano has been deeply eroded, 
and a central valley reveals a core of coarse-grained rocks: gabbros and 
peridotites, which represent the lavas that solidified in the conduit of 
this volcano when it became extinct. No syenites were found among 
the rocks, although trachytes and phonolites are common among the 
latest lavas in the upper, or marginal, parts of this volcano. Such 
alkalic feldspathic lavas are clearly differentiates of the basaltic magma, 
the complimentary varieties being peridotites and peridotitic lavas 
which are characteristic of these and neighboring volcanoes. 
Of the chemical analyses of Tahitian rocks already published by La- 
croix two are of preponderating varieties of basalt, and two are of pre- 
ponderating varieties of gabbro from the central core in Papenoo Valley. 
The four analyses agree closely with one another and their average may 
be considered a first approximation to what was probably the parent 
magma from which all the lavas of Tahiti were derived. The estimated 
specific gravity of a completely crystallized rock of such a composition 
is 3.18. The average of the relative densitites of 4 specimens of gab- 
bros and peridotites from the Papenoo Valley is 3.18, and of 4 similar 
rocks from the central valley of Taiarapu is 3.16. These estimates 
leave out of account the syenites, trachytes and phonolites of the island, 
for the amount of these rocks when compared with the bulk of the 
basaltic volcanoes is almost negligible. 
This preHminary estimate of the relative density of the soHdified 
magmas of these two volcanoes may be slightly too high, and is some- 
what greater than those calculated for the soHdified magmas of Hawaii 
and Reunion. It clearly indicates that the magma from which the 
Tahitian volcanoes sprang is sufficiently heavy to correspond to the 
great depth of the Pacific Ocean in this region according to the theory 
of isostasy. 
Moorea, or Eimeo, is an island volcano ten miles northwest of Tahiti, 
which has been greatly eroded and partly submerged under the sea. 
The central portion has been reduced to a deep valley surrounded by 
high precipitous mountains 2500 to 3500 feet in height. The upper 
portions are composed of horizontal layers of basalt lava. The central 
basin is drained northward by two valleys, drowned by the sea, form- 
ing bays with a high mountain ridge between. Small, radial, valleys 
cut the main ridge into short spurs which pitch steeply to the coast. 
The exposed lava sheets are seen to dip away from the center of the 
