PETROLOGY: IDDINGS AND MORLEY 
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no. 17, and the basalt from Huahine, no. 16, are very similar chemically, 
while the basalt from the base of Maura tapu on Huahine, no. 18, is lower in 
magnesia and higher in lime and alumina. These basalts do not contain 
normative nephelite, and differ from one another somewhat in texture. 
From the foregoing it is seen that the basaltic rocks of this region, and their 
coarsely crystallized phases, which occur in the cores of the volcanoes of 
Tahiti and Taiarapu, are normatively nephelite-bearing, except some of the 
highly olivinitic varieties, and some others. Nephelite js visibly present in 
the modes of the coarsely crystallized rocks, and is possibly present in micro- 
scopic crystals in many of the fine-grained and aphanitic basalts, though it is 
probable that it is represented by analcite in some instances, either as a pri- 
mary mineral, or as a product of alteration. 
The trachytic and phonolitic lavas, which are known to occur at five of the 
volcanic islands visited, are very similar to one another chemically, as is 
shown by analyses 1 to 8. The rocks from Nutae, no. 3, and Point Riri, 
no. 6 on Taiarapu, are light gray and but slightly porphyritic, with fissile part- 
ing and satin lustre. The first occurs as boulders on the beach associated with 
hauynophyre. The second is in place, and is exposed in large blocks. Each 
contains a small amount of normative nephelite, and a little that can be iden- 
tified as modal nephelite, so that the rocks are properly nephelite-bearing 
trachytes, rather than phonolites. They have a microtrachytic texture, the 
first one containing small scattered phenocrysts of alkalic feldspar. Similar 
rocks occur on Huahine, nos. 1 and 5. They are darker colored and more 
fissile. Microscopically they appear to contain abundant minute crystals of 
nephelite, and to be characteristic phonolites. However, most of the rect- 
angular crystals are alkalic feldspars and not nephelite, as their index of re- 
fraction shows. These rocks also are nephelite-bearing trachytes and not 
properly phonolites. The same is true of similar massive rocks from Raiatea, 
the fissile mass forming the top of Mount Tapioi, no. 4, and the rock of the 
sugar-loaf dome on the east side of the island, no. 2. These rocks are nephelite- 
bearing trachytes with small amounts of nephelite. The rock from the top 
of Mount Tapioi contains minute crystals of what appear to be sodalite scat- 
tered through the feldspars. Similar nephelite-bearing trachytes with less 
nephelite form large bodies of rock on Moorea, the analysis of one of them 
being given in no. 8. 
Some varieties of these trachytic rocks, occurring on Taiarapu, contain 
hauynite in small crystals, and 12% of normative nephelite and are properly 
phonolites. Analysis 7 is from such a rock. One variety, no. 11, from the 
beach at Tautira consists of alkalic feldspar and andesine, with some nephelite 
and sodalite, and contains abundant small phenocrysts of brown hornblende, 
with much titanite and few augites and micas. Chemically it is very similar 
to a tephritic trachyte from Bauza, Columbreta, described by Becke. Its 
symbol in the Quantitative System of Classification shows that it is transi- 
tional. In the qualitative system it corresponds to a nephelite-latite, or 
