222 
PACIFIC SCIENCE, Vol. II, July, 1948 
and deformation in the volcanic series and the 
absence of highly elevated and tilted limestones 
of Plio-Pleistocene age suggest that the plateau 
has been a relatively stable unit since the middle 
part of the Tertiary period. 
In the southern Mariana Islands the Tertiary 
and early Quaternary limestones have been 
raised as much as 1,500 feet above the present 
sea level (Rota). Locally the strata have been 
highly tilted and the surfaces of marine ter¬ 
races have been offset and tilted by faulting 
(Guam, Rota, Tinian, Saipan). Moreover, the 
volcanic rocks underlying these limestones have 
been strongly folded as well as faulted (Guam, 
Saipan). All evidence at hand indicates that 
diastrophic forces were active in the Mari¬ 
ana Islands throughout the Tertiary, and that 
they are still going on. In the Palau Islands 
the uplift of the Plio-Pleistocene limestones 
amounts to at least 700 feet (Urukthapel 
Island ), and in the Admiralty group, limestones, 
presumably of the same age, have been raised 
at least 200 feet (Manus). The volcanic rocks 
upon which these limestones rest have been 
moderately folded and faulted, but there is no 
evidence of regional metamorphism in the rocks 
seen in either of these groups. 
The Yap ridge appears to be older, and, at 
the present time, more stable than either the 
Mariana or Palau ridges. No raised limestones 
have been found on Yap, and in this respect the 
group resembles the high islands of the eastern 
Carolines. The position of Yap with respect 
to major structural features, together with the 
fact that most of the rocks exposed on its 
islands are well metamorphosed and are of 
typical continental varieties, is responsible for 
placing it west of the continental line. 
Note: After this paper was transmitted my 
attention was called to two other papers, one 
botanic (Selling, 1947), one geologic (Bryan, 
1945), which contain maps showing the posi¬ 
tion of one or more lines in the western Pacific 
Ocean. A third and highly important paper 
(Hess, 1948) appeared while this article was 
in press. Although no mention of these papers 
is made in the text they are included in the list 
of references. 
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Born, A. 1933. Der Geologische Aufhau der 
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Daly, R. A. 1916. Petrography of the Pacific 
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Chap. XII in Physics of the earth — VII. 
Internal constitution of the earth, x-J-413 
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Hess, Harry H. 1948. Major structural fea¬ 
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Tayama, R. 1939. Terraces of the South Seas 
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——— 1946. Bathymetric chart, Korea to New 
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