Schist on Truk— Bridge 
219 
1 and 4). The general form of the mountain 
agrees quite well with Kramers description, the 
upper slopes are only sparsely forested, and 
there is a wagon road along the eastern spur 
which reaches the summit. This is the only 
one of the three high peaks on Moen which is 
reached by a road. The slopes of the upper 
half of Witipon are covered with a dense 
growth of coarse grass, ferns, low brush, and 
scattered trees, and stand in sharp contrast to 
the densely forested slopes of the other peaks 
on the island (Fig. 1). Thus there appears to 
be little doubt of the correct identification of 
the region described by Kramer. 
In the course of the investigations, Witipon 
was climbed on two different occasions, rock 
samples were collected at several localities (Fig. 
4), and it is believed that a representative 
suite was obtained. The lower part of the moun¬ 
tain is composed of a series of basalt flows. 
Two distinct types, one porphyritic, the other 
non-porphyritic, may be recognized in hand 
samples. These basalts are capped by what 
appears to be a thick bed of well-stratified tuff, 
but which has actually been determined as a 
trachytic flow. This flow is at least 300 feet 
thick. The entire series dips north at low 
angles, commonly less than 5 degrees. 
It is believed that this trachytic flow, repre¬ 
sented in the collections by specimens TK-21, 
22, and 24, 4 is the rock which Kramer identi¬ 
fied as schist. The fine, parallel, flow structure, 
and the glistening, micaceous-appearing, freshly 
broken surfaces, particularly those which are 
parallel to the flow lines, give this rock a 
schistose appearance, so that superficially it 
resembles the chlorite and amphibolite schists 
found on Yap. However, it is much lighter in 
color, and its mineralogic composition and tex¬ 
ture as seen in thin sections (Fig. 2) are totally 
different. According to G. A. Macdonald (per¬ 
4 The specimens described in this article, together 
with rocks collected from other islands visited on this 
trip, are now in the laboratories of the U. S. Geo¬ 
logical Survey, Washington, D. C. They will eventu¬ 
ally be deposited in the U. S. National Museum. A 
duplicate set is already on deposit in the Bernice P. 
Bishop Museum in Honolulu. 
sonal communication, 1948), the Hawaiian 
trachytes and oligoclase andesites commonly 
have well-developed, thin, tabular feldspar plates 
oriented parallel to the flow planes. The sur¬ 
faces of these plates often exhibit a micaceous- 
appearing sheen, caused by the parallel orienta¬ 
tion of innumerable tiny feldspar grains, caus¬ 
ing the rock to resemble a fine-grained mica 
schist. This explanation applies equally well to 
the Truk specimens. 
No other occurrence of this trachytic material 
was found, either on Moen or on the other 
islands which were visited in the Truk group. 
However, this does not rule out the possibility 
of other occurrences, for the examination was at 
best a hasty reconnaissance and many of the 
large islands were not visited. 
Mt. Chukumong (Teroken, on H. O. Chart 
6047), 1,214 feet, the highest peak on Moen 
Island, lies about 1.25 miles west of Witipon. 
If the regional strike and dip of the trachytic 
material continue westward without apprecia¬ 
ble change, this material should cap the summit 
of Mt. Chukumong, and should also appear in 
the slopes of Mt. Tonaachau (794 feet), the 
conspicuous peak on the northwestern tip of 
Moen Island (Fig. 1). No definite information 
about the kind of rock on its summit is avail¬ 
able. However, the upper slopes are covered 
with a dense forest, totally unlike the grassy 
upper slopes of Witipon, and this suggests that 
the summit may be underlain by a different type 
of rock. Mt. Tonaachau was climbed from two 
different sides and only basalts and basaltic 
agglomerates were seen. Macdonald (personal 
communication, 1948) says that trachytic flows 
which he has observed on other oceanic islands 
are commonly extremely localized, and that no 
special structural assumptions are necessary to 
account for the local distribution of this ma¬ 
terial. 
The western and northwestern shores of the 
island were examined and no indications of 
metamorphic rock were found. It is possible, 
but in the writer’s opinion highly improbable, 
that such rocks occur along the southern and 
southeastern coasts, which were not studied. 
