220 
PACIFIC SCIENCE, Vol. II, July, 1948 
PETROGRAPHY 
The specimens collected on Moen Island have 
been examined by Miss Jewell J. Glass of the 
U. S. Geological Survey, and the following 
quotations are taken from her report: 
TK-19, TK-20, black, porphyritic volcanic 
rock. Identified as: Porphyritic basalt. This 
rock is fresh and represents a typical example 
of porphyritic olivine basalt. The phenocrysts 
consist of unaltered crystals of olivine, augite, 
and bytownite. The groundmass consists of a 
net-work of feldspar (plagioclase) laths, fine¬ 
grained augite and olivine, with abundant octa- 
hedra of magnetite. 
TK-17, TK-18, dense, black volcanic rock. 
Identified as: Fine-grained basalt. Different in 
texture but the same in composition as TK-19 
and TK-20 (described above). The ground- 
mass consists of closely packed laths of plagio¬ 
clase, fine grains of olivine, swarms of black 
granules of magnetite and a few phenocrysts of 
augite. In TK-18 the olivine plates have altered 
to iddingsite. 
TK-24, pale gray, fine-grained rock contain¬ 
ing a scattering of feldspar phenocrysts. Identi¬ 
fied as: Trachy tic rock . This is an extrusive 
igneous rock. In thin section it shows, in the 
crystallized groundmass, typical trachytic struc¬ 
ture. The groundmass consists essentially of 
minute lath-shaped feldspar crystals having a 
distinct parallel arrangement, or flow pattern, 
and a sprinkling of tiny granules of magnetite. 
The tabular feldspar phenocrysts show a zonal 
banding and indications of multiple twinning. 
The irregular optical properties of the feldspar 
indicate that it has undergone some internal 
structural change. Minute flakes of biotite and 
chlorite are scattered throughout the mass, and 
a few remnants of resorbed biotite crystals 
remain. A small percentage of a brownish ma¬ 
terial occupies spaces between the feldspar 
laths. Some of it resembles glass, but most of 
it is birefracting and appears to be fine-grained 
aggregates of ferromagnesian minerals. Occa¬ 
sionally a minute grain of augite is observed. 
TK-21. This specimen is the same type of 
rock as TK-24. However it has been considerably 
altered by weathering. A crushed sample of the 
more altered portion of the rock contains a 
platy or apparently micaceous mineral, roughly 
hexagonal in outline, and which is an alteration 
product of undetermined composition. 
TK-22, cream-colored, soft, chalk-like ma¬ 
terial. Identified as: Completely altered prod¬ 
uct of the trachytic rocks described above. Ther¬ 
mal analysis was not conclusive, neither was the 
x-ray pattern. Suffice it to call the material 
clay until more work can be done. 
TK-23 consists of reddish-tan, irregularly 
shaped, nodular material, associated with clays 
(TK-23-a). These overlie the trachytic material 
and are believed to have been derived from it. 
The nodules have been determined to be 
bauxite. 5 
CONCLUSIONS 
The placing of the continental line involves 
two general classes of factors: (1) differences 
in the kind and composition of the rocks form¬ 
ing the islands and (2) differences in struc¬ 
tures. The latter is possibly the more important 
and may be further subdivided into: (a) broad 
major structures reflected by the larger features 
of the ocean bottom and (b) lesser structural 
features which may be observed in the rocks 
exposed on the various islands in question. 
The importance of any set of criteria varies 
greatly from place to place, and all of the 
above factors have been used in the present 
relocation of the lines. 
In view of the identification of Kramer’s 
material as a trachytic flow and of the observa¬ 
tions made on Moen and on many of the adja¬ 
cent islands, the occurrence of metamorphic 
rocks on Truk now seems highly improbable. 
The rocks collected on Truk are quite similar 
to those found on Ponape and Kusaie, and 
quite different from those observed on the high 
islands to the west and southwest, a fact already 
noted by Yossii (1937: 74, Tables I, II), who 
lists trachyte from Ponape, but not from Truk 
or Kusaie. This suggests that these three island 
groups (Truk, Ponape, and Kusaie) are the 
deeply eroded summits of a group of volcanoes 
rising above the submarine Caroline plateau, 
and that they are more nearly analogous to the 
Hawaiian volcanoes than to the various island 
groups lying to the west and southwest. There 
is, therefore, no petrologic reason for including 
5 See "On the Occurrence of Bauxite on Truk” on 
page 223 of this issue. 
