A Restudy of the Reported Occurrence of Schist on Truk, 
Eastern Caroline Islands 1 
Josiah Bridge 2 
INTRODUCTION 
The islands of the western Pacific Ocean are 
commonly grouped into two main divisions: 
continental and oceanic. The former are be¬ 
lieved to be the remnants of a continental area; 
the latter are thought always to have been 
islands. The distinction is based upon several 
facts, among which are the types of rocks of 
which the islands are composed, the major 
structure lines of the ocean basins, and the 
structures of the islands themselves. The loca¬ 
tion of the eastern border of the former Mela¬ 
nesian or Australasian continent has been the 
subject of considerable speculation. A number 
of authors have presented "lines” showing the 
distribution of different rock types, structural 
data, and other geologic information in the 
western Pacific islands, and from these have 
made various deductions about the former extent 
of the Melanesian continent. Among these are 
the "border of the South Pacific Basin” (Mar¬ 
shall, 1912), the "andesite line” (Born, 1933), 
the "metamorphic-plutonic” and "probable 
boundary of the former Melanesian continent” 
lines (Ladd, 1934), and the "Sial line” (Stearns, 
1945; 1946). Inasmuch as many of the islands 
in the vicinity of these "lines” were closed to 
all except Japanese geologists from about 1916 
until quite recently, most of the evidence for 
their position and the subsequent classification 
of the islands on either side of them as con¬ 
tinental or oceanic has been derived largely 
from a study of the older literature. It is the 
purpose of the present paper to revise a por¬ 
tion of the "metamorphic-plutonic” and "con¬ 
tinental boundary” lines of Ladd in accordance 
with some recent observations made on Truk, 
1 Published by permission of the Director, U. S. 
Geological Survey. 
2 Geologist, U. S. Geological Survey. Manuscript 
received February 11, 1948. 
one of the key island groups determining the 
position of these lines. 
Acknowledgments: The writer is deeply in¬ 
debted to Miss Jewell J. Glass and C. S. Ross 
for the petrographic examinations and descrip¬ 
tions of the various rocks and for the photo¬ 
micrograph (Fig. 2). Thanks are also due to 
G. A. Macdonald, H. S. Ladd, and J. I. Tracey, 
all of whom have read and criticized the manu¬ 
script and have offered many helpful suggestions. 
PREVIOUS WORK 
Ladd (1934: 51, Fig. 6) gave a brief de¬ 
scription of the former Melanesian continent 
and outlined on a map of the Pacific Ocean 
south of lat. 10° N. the probable eastern boun¬ 
daries of several types of rocks, as well as the 
probable eastern boundary of the former Mela¬ 
nesian continent. His map, with some modifi¬ 
cations, is reproduced as Figure 3 in this paper. 
In the original figure, one of the lines, desig¬ 
nated the "metamorphic-plutonic line,” starts 
just east of Yap, extends southeastward around 
the Truk group, then turns abruptly south to 
New Ireland and thence southeastward along the 
northeastern edge of the Solomon Islands. A 
second line, hereafter referred to as the "con¬ 
tinental line,” marks the "probable former 
boundary of the Melanesian continent and true 
structural boundary of the Pacific Basin” and 
lies just east of and roughly parallels the "meta¬ 
morphic-plutonic line.” Both lines were drawn 
so as to include Truk among the continental 
islands and to exclude Ponape and Kusaie, high 
volcanic islands lying east of Truk and geo¬ 
logically similar to it. 
The inclusion of Truk among the continental 
islands was based on the reported occurrence 
of schist on one of the islands of the group 
(Daly, 1916). Dalys paper, in turn, was not 
based on direct observation but on information 
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