112 
PETROLOGY: IDDINGS AND MORLEY 
A basalt still richer in olivine, of the utmost freshness occurs on the east 
side of Raiatea, its analysis, no. 24, shows 23.97% of magnesia, or nearly 45% 
of normative olivine. It is a most beautiful chrysophyre, or limburgite, with a 
holocrystalline groundmass crowded with microscopic augites, olivines, mag- 
netite and subordinate plagioclase. On the Island of Moorea a limburgite 
with similar chemical composition, no. 23, was found in large blocks, but 
not in place. It is more coarsely crystallized and resembles a peridotite 
megascopically. It is perfectly fresh and has a scant matrix of microscopic 
plagioclase. The norms of these highly olivinitic basalts do not contain 
normative nephelite, but contain normative hypersthene. The silica is com- 
paratively high for the amount of alumina, which is low; the magnesia being 
abnormally high. 
In the central core of the dissected volcano of Tahiti, and in that of Taia- 
rapu, there are coarse-grained theralites which are chemically similar to the 
non-porphyritic basalt forming the upper flow at Point Tapahi, Tahiti. Their 
analyses are nos. 20, 21, and 22. They are limburgose and limburgose-etin- 
dose, and are characterized by notable amounts of normative nephelite, which 
is also modal. In no. 22 there is also normative leucite, which, however, does 
not appear in the mode, which contains considerable biotite. The theralite 
from Vaitipihia, Tautira Valley, Taiarapu, no. 20, is rich in augite, brown 
amphibole and biotite, with subordinate plagioclase and nephelite. That 
from Maroto River, in the Papenoo Valley, Tahiti, no. 21, is very much like 
no. 20, but has less amphibole and a slightly different texture. The other 
theralite from Vaitipihia, no. 22, contains large brown amphiboles, sur- 
rounded by brown biotite, with little augite, and much apatite. The chemical 
compositions of coarse-grained rocks of unusual mineral composition from the 
core of Taiarapu are given in nos. 29 and 30. The first is a narrow vein of 
pyroxenic rock traversing basalt. It consists almost wholly of slender den- 
dritic crystals of augite in a microscopically fine-grained matrix. The norm 
has 45% of diopside, 5% of olivine, 26% of anorthite and small amounts of 
normative nephelite and leucite. The second rock, no. 30, is a peridotite 
composed of augite, olivine, brown hornblende, iron ores and pyrite, with very 
small amounts of feldspathic minerals; the norm containing a little normative 
nephelite and leucite. 
The analyses of other basalts from these islands are given in nos. 14 to 
18 in the table. They are camptonose and camptonose-auvergnose, and 
are from Moorea, Huahine, Tahaa and Bora Bora. The basalt from Tahaa, 
no. 14, is rather coarse-grained and forms a dike near the coast on Rei Point. 
It is the rock used for ballast by small boats in this region, and may be found 
in the ports on various islands, and is probably the rock called granite by early 
explorers. It is a dolerite and looks like a fine-grained gabbro. It is rich in 
olivine and its norm contains a small amount of nephelite. The basalt from 
Bora Bora, no. 15, is porphyritic, very rich in olivine, and has nearly the 
same chemical composition as no. 14. The columnar basalt from Moorea, 
