110 
PETROLOGY: IDDINGS AND MORLEY 
A CONTRIBUTION TO THE PETROGRAPHY OF THE SOUTH SEA 
ISLANDS 
By J. P. Iddings and E. W. Morley 
Brinklow, Maryland, and West Hartford, Connecticut 
Communicated February 18, 1918 
A brief statement of the geological structure and general character of the 
rocks of the Islands of Tahiti, Moorea, and the Society Group has been given 
in a previous number of the Proceedings of the National Academy, from which 
it appears that each island is a profoundly eroded volcano, consisting mainly 
of basaltic lavas rich in olivine and augite, with inconspicuous feldspar, and 
that at five of the volcanic islands there are trachytic or phonolitic lavas which 
have been erupted late in the period of activity. In two volcanoes erosion has 
exposed coarsely crystallized cores of gabbroic and theralitic rocks with peri- 
dotites, and in one case syenites and nephelite-syenites as the latest eruptions. 
Nearly seventy years ago J. D. Dana called attention to these syenitic 
rocks on Tahiti, and remarked that they were only a feldspathic variety of the 
same igneous rocks that constitute the island. Eight years ago Lacroix pub- 
lished a description of the alkalic rocks of Tahiti with chemical analyses, lay- 
ing particular stress on the syenitic varieties and on the haiiynophyres, with 
certain limburgitic lavas, but noting the fact that the preponderant rocks of 
the islands are basalts rich in olivine. From the emphasis laid upon the alkalic 
rocks, one gets the impression that they are more abundant than is actually 
the case. However, their theoretical importance has not been exaggerated. 
More recently Marshall has analysed and described phonolitic rocks from the 
Leeward Islands and from Raratonga, Cooks Islands, and has analyzed sev- 
eral rocks from Tahiti. So there are already a number of chemical analyses 
of igneous rocks from the islands of this part of the Pacific Ocean. 
In order to extend the investigation somewhat further, and to include the 
more common varieties of basalt so as to give a clearer idea of the prevailing 
rocks of the islands, chemical analyses have been made of rocks from different 
islands of the Georgian and Society groups. These have been placed in 
sequence in one table for comparison with one another, and to show the 
resemblances among the various lavas of these islands. The analyses have 
been obtained in part with the aid of grants 192 and 203, from the Bache Fund 
of the Academy. That is, those by Professor Foote and by Dr. Washington. 
The microscopical study has also been carried on with the aid of these grants. 
As the specimens collected represent over 550 rocks from seven volcanic 
islands, it is only possible in this preliminary statement to notice particularly 
the 30 specimens whose analyses are published for the first time in the accom- 
panying table, with some observations concerning their relations to the rocks 
with which they are associated. 
The rocks of Tahiti are almost wholly basalts rich in olivine and augite, 
