PETROLOGY: 1DDINGS AND MORLEY 
111 
with few or no phenocrysts of feldspar. They differ from one another some- 
what in the size and abundance of the phenocrysts of olivine and augite. 
At one extreme are basalts with abundant phenocrysts; at the other are ba- 
salts without any, but with ill-defined spots which are lighter colored than 
the body of the rock. Such basalts form large flows often in superposition, 
as at Point Tapahi on the north coast where a strongly porphyritic basalt 
forms the lower layer or flow, at the water's edge, and a non-porphyritic 
spotted basalt forms the upper flow. The upper rock has been analyzed, 
no. 19 in the table, and proves to be a limburgose, bordering on camptonose, 
with 8.5% of normative nephelite. The light-colored spots in the rock are 
probably due to areas of altered nephelite, or to analcite. The rock is apha- 
nitic, and under the microscope is seen to be holocrystalline; composed of 
augite, magnetite, ilmenite, and olivine in a matrix of plagioclase with nephe- 
lite or analcite. Similar basalt forms a massive flow and has been quarried 
near Papeete. A strongly porphyritic basalt rich in olivine, like the rock at 
Point Tapahi, is in place on the road farther west. Its analysis, no. 27, shows 
it to be uvaldose, a highly mafic rock without normative nephelite. It is 
holocrystalline; the groundmass crowded with augite, olivine and magnetite, 
with quite subordinate amount of plagioclase feldspar. It is in fact limbur- 
gite. These two varieties of basalt are common throughout the islands 
visited, a very similar rock, no. 28, having been analyzed from the west coast 
of Raiatea. It is, however, somewhat more coarsely crystalline, the indi- 
vidual crystals being distinctly visible microscopically. Some of the plagio- 
clase feldspars have outer zones of alkalic feldspar, probably soda-orthoclase. 
A limburgitic basalt which appears to be a large massive body, exposed in 
Fautaua Valley on the trail above the waterfall, is a gray rock with small 
miarolitic cavities. It is holocrystalline and consists of abundant subhedral, 
violet-tinted augites, of variable sizes, with much colorless olivine, subhedral 
magnetite, in part dendritic, and areas of poikilitic plagioclase, with patches 
where the matrix is alkalic feldspar and analcite. The chemical analysis is 
no. 26 and the norm is relatively high in alkalic feldspar, low in anorthite, 
with 7% of normative nephelite. There is 5% of apatite which appears as 
thin acicular prisms. A corresponding amount of apatite occurs in a theralite 
from Taiarapu, analysis 22. Another basalt from Fautaua Valley is found in 
boulders in the stream near the ridge above the falls. Its chemical com- 
position, no. 25, is similar to the rock just described, no. 26, but it appears 
somewhat differently under the microscope. It consists chiefly of violet- 
tinted augite, with abundant colorless olivine, and magnetite, and clusters of 
rods of ilmenite, with minute needles of augite. Between these is a small 
amount of colorless matrix which is in part plagioclase feldspar. The norm 
shows a small amount of anorthite, 19% of feldspathoids, and no alkalic feld- 
spar. There is about 2% of calcium orthosilicate which does not appear as 
melilite in the mode, and is probably incorporated in the mafic minerals. 
This is also a limburgite. 
