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PACIFIC EXPLORATION: J. P. IDDINGS 
characteristics which distinguish them from series of rocks in other 
regions. In short, that the series of igneous rocks in various parts 
of the world are not exactly alike, though some may be. 
Recently I have called attention to the apparent relation between 
regional characteristics of igneous rocks and the broad demands of the 
theory of isostasy. The igneous rocks of the North American con- 
tinent, both as a whole and when considered in regional groups, such as 
those of the Pacific cordilleras, the Colorado plateau, or the Atlantic 
coast, average lighter than the igneous rocks of islands in the deep oceans 
such as Hawaii in the Pacific, and Reunion in the Indian Ocean. The 
relative densities of a completely crystallized average magma of the 
Pacific Coast rocks, and those of average magmas for Hawaii and Reunion 
are to one another as 2.85 to 3.12, for each of the island groups of rocks. 
This appears to be in accord with the demands of isostasy, if igneous 
rocks represent the material forming deep portions of the lithosphere, 
or material immediately below it, or at its base. However, much 
remains to be done in studying the igneous rocks of all parts of the earth, 
especially of the little known islands of the Pacific Ocean. 
It was with the hope of contributing something to the solution of the 
problem of isostasy from the petrographic side that I undertook recently 
a reconnaissance of some volcanic islands in the South Pacific, namely: 
Tahiti, Moorea, the Leeward Islands: Huahine, Raiatea, Tahaa and 
Bora Bora; besides the two largest islands of the Marquesas, Hiva-oa 
and Nukahiva. 
So long ago as 1849 James D. Dana pubHshed an excellent description 
of Tahiti, and mentioned the occurrence of a variety of syenite among 
the basalts of this volcano, which he considered to be ''only a feldspathic 
variety of the same igneous rocks that constitute the island;'' a per- 
fectly correct assumption. Darwin visited Tahiti a few years eariier 
than Dana and noted the basaltic character of the rocks. In a conver- 
sation with the writer 30 years ago Dana remarked 'Tf you want to 
study a dissected volcano, go to Tahiti." At that time it seemed a dif- 
ficult thing to do. Today the island is on a direct mail route between 
San Francisco and Wellington, N. Z. 
In 1910 Lacroix published a description of Tahitian rocks which 
were in the Museum of National History in Paris. Besides abundant 
basalts the collection embraced coarsely crystallized gabbros, syenites 
and lavas that are characterized by nephelite and haiiynite, minerals 
generally considered absent from igneous rocks occurring in the region 
of the Pacific Ocean, but already found to some extent in the Hawaiian 
and Samoan Islands. 
