PACIFIC EXPLORATION: J. P. IDDINGS 
415 
Professor Marshall, of Dunedin, N. Z., has described nephelite-b earing 
rocks from the Cook Islands and from some of the Leeward Islands in 
the Society group; and in 1915 he pubHshed an account of his journey 
to the center of the island of Tahiti. From these descriptions it was 
known that all of these islands are basaltic and that some of them 
contain trachytes and phonolites, but the relative abundance of these 
rocks and the more specific characters of the basaltic lavas were not 
definitely known, and no approach to an estimate of the average mag- 
mas of the several islands could be gained from the fragmentary nature 
of the information at hand. 
A reconnaisance of the islands was thought to be worth the effort, 
and has been productive of valuable results. However, a great deal 
remains to be done in the way of more accurate detail exploration of 
the islands than I was able to carry on in the few months at my disposal, 
and with the limited means at my command. 
Tahiti, the largest island in this region, is an extinct volcano, deeply 
eroded by streams that have cut canyon-like valleys, which radiate 
from a circular range of high mountains, surrounding the deep central 
basin heading Papenoo valley, which drains northward. This central 
basin is 5 or 6 miles across from crests of the encircHng range with its 
peaks of 4000, 5000, 6000, and in Orofena of 7000 feet in altitude. 
In the geometrical center of the island, which is in the western head 
of the Papenoo Valley, there is a low conical hill, Ahititera, composed 
of coarsely crystallized rocks, gabbros, peridotites with subordinate 
amounts of nephelite-syenite and other rocks. The surrounding rocks, 
so far as seen through the forest of tropical vegetation, are basaltic 
tuff breccias and basaltic lava flows, the latter preponderating in the 
upper parts of the mountains and in the outlying spurs and slopes of 
the ancient volcano. 
The top of the central hill, Ahititera, is about 2500 feet above sea 
level, and may be assumed to be about 7000 feet below what was atone 
time the bottom of the final crater of the volcano, if the mountain had 
the same profile as the great modern volcanoes of Hawaii. The valley 
bottoms are nearly level for miles inland from the coast, and their 
heads lie deep below the summits of the mountain ridges between them. 
Their sides are very steep, in many places almost vertical walls, thinly 
covered with clinging ferns and vines. 
The lavas composing the Tahitian volcano are basalts rich in ferro- 
magnesian minerals, which commonly appear as prominent crystals. 
Feldspars are seldom seen, but are prominent in some varieties of the 
basaltic lavas. There are very few lavas with a trachytic appearance 
