Schist on Truk—B ridge 
221 
the islands of this group with the continental 
islands. 
Structural evidence also favors the exclusion 
of the Caroline submarine plateau from the 
continental area. Ladd’s original lines (Fig. 3), 
drawn to outline occurrences of metamorphic 
and plutonic rocks, cross this plateau between 
Truk and Ponape, thereby placing Truk and 
the western half of the plateau in the con¬ 
tinental area and leaving Ponape, Kusaie, and 
the eastern half in the oceanic area, while 
Stearns (1945, 1946) places the entire Caroline 
group in the continental area. A study of exist¬ 
ing bathymetric charts (Int. Hydrographic 
Bureau, 1940) shows little in the configuration 
of the ocean bottom to favor such a division. 
The Caroline plateau is shown as a number 
of irregular elevations rising above the 5,000- 
meter contour and not separated from the basins 
on either side by deep trenches or other indica¬ 
tions of major structural features. However, it 
must be recognized that most of the bathy¬ 
metric charts now available have been con¬ 
structed from insufficient data and that the 
topographic features shown on them are at best 
only approximations. The most reliable chart 
of recent date (H.O. chart 5485) does not 
extend far enough east to include the eastern 
Carolines. A comparison of this chart with 
earlier ones shows many important refinements 
in ocean-bottom topography which have been 
made possible by the inclusion of new data. 
The "metamorphic-plutonic” and 'continen¬ 
tal boundary” lines, as here drawn (Fig. 3), for 
the most part follow major structural features. 
These are the West Caroline and Palau trenches, 
profound deeps with maximum depths of 4,500 
and 3,500 fathoms, which lie just east of and 
parallel to the Yap and Palau ridges (H.O. 
chart 5485). Southwest of the Palau Islands 
the lines swing abruptly southeast along the 
northeast base of the submarine ridge on which 
the Solomon and Admiralty Islands are located 
and eventually join the continental lines as 
drawn by Marshall (1912) and Ladd (1934). 
No well-marked structural trench is present 
along the base of this ridge, the average depths 
being 2,000 to 2,500 fathoms, with only a few 
points exceeding the higher figure. However, 
there is a marked break in slope at the base 
of this ridge, which is in direct prolongation 
with the trend of the great Mindanao trench, 
and this suggests that it might be the diminish¬ 
ing extension of a major structural line. The 
northeastern slope of the Admiralty-Solomon- 
Island ridge is similar to the northern front of 
the Caroline plateau, having about the same 
degree of slope although the latter descends 
to somewhat greater depths. On the basis of 
the known configuration of the ocean bottom 
as shown on the existing bathymetric charts 
there is little choice between the continental 
line as here drawn and the "Sial line” of Stearns 
(1945, 1946). The distinction must be made 
on other evidence. 
Final reasons for placing the "metamorphic- 
plutonic” and "continental lines” so as to ex¬ 
clude the Caroline submarine plateau from the 
continental area are the absence of extensive 
deformation and evidence of recent uplift in 
the rocks of the high islands which rise above 
it, a striking contrast to conditions on the high 
islands lying to the west of the "continental 
line.” 
On Truk, Ponape, and Kusaie there are no 
metamorphic rocks, and there is little or no 
evidence of extensive folding or faulting. Such 
dips as may be seen in the various lava flows 
are gentle, and may well be original. There are 
no strongly elevated and tilted limestone ter¬ 
races, such as are common in the Mariana 
Islands and to a lesser extent in the islands of 
the Palau and Admiralty groups. In fact, the 
occurrence of limestone on Truk, Ponape, and 
Kusaie is limited to the low, discontinuous, 5- 
to 7-foot bench which partially surrounds most 
of the high islands. On Truk, Ponape, and 
Kusaie there are indications of topographic 
benches at relatively high elevations. These 
have been interpreted by Tayama (1939) as 
terraces of marine origin, older than the sur¬ 
face upon which the Plio-Pleistocene limestones 
of the Mariana, Palau, and Admiralty groups 
were deposited. The absence of metamorphism 
