190 
NATURE 
[APRIL 15, 1915 

ment of Science, with an invitation to attend its meet- 
ing in Australia last August as a foreign guest, 
enabled me to spend the greater part of the year 1914 
‘in visiting a number of islands in the Pacific Ocean 
with the object of testing various theories that have 
been invented to account for coral reefs. Thirty-five 
islands, namely, Oahu in Hawaii, eighteen of the 
Fiji group, New Caledonia, of which the entire coast- 
line was traced, the three Loyalty islands, five of the 
New Hebrides, Raratonga in the Cook group, and 
six of the Society islands, as well as a long stretch 
of the Queensland coast inside of the Great Barrier 
reef, of north-eastern Australia, were examined in 
greater or less detail. A brief statement of my re- 
sults has been published in the Proceedings of the 
National Academy of Sciences for March, 1915. A full 
report upon my observations will appear later, prob- 
ably in the Bulletin of the Museum of Comparative 
Zoology at Harvard College. The general conclu- 
sions reached are here briefly summarised. 
Any one of the eight or nine theories of coral reefs 
will satisfactorily account for the visible features ‘of 
sea-level reefs themselves, provided the postulated 
conditions and processes of the invisible past are 
accepted; hence a study of the visible features of the 
reefs alone cannot lead to any valid conclusions. 
Some independent witnesses must be interrogated, in 
the hope of detecting the true theory of their origin. 
The only witnesses, apart from sections obtained by 
deep and expensive borings, available for sea-level 
reefs are the central islands within oceanic barrier 
reefs, or the mainland coast within a continental 
barrier reef. The testimony of these witnesses has 
been too largely neglected, apparently because most 
investigators of coral reefs have been zoologists, little 
trained in the physiography of shore lines. Elevated 
reefs afford additional testimony in their structure and 
in the relation of their mass to its foundation; but 
this testimony also has been insufficiently considered, 
perhaps because most investigators of reefs have as 
zoologists been little trained in structural geology; 
hence it seemed desirable to give as much time as 
possible on the Pacific Islands to questioning the 
independent witnesses above designated. ; 
The testimony of the first group of witnesses—the 
central islands of barrier reefs—convinced me _ that 
Darwin’s theory of subsidence is the only theory 
competent to explain not only the development of 
barrier reefs from fringing reefs, but also the shore- 
line features of the central islands within such reefs; 
for the embayments of the central islands testify 
emphatically to subsidence, as Dana long ago pointed 
out; thus my results in the study of this old problem 
of the Pacific agree with those of several other recent 
students, especially Andrews, Hedley, and Taylor of 
Australia, and Marshall of New Zealand. Darwin’s 
theory of subsidence also gives by far the most prob- 
able explanation of atolls; for it is unreasonable to 
suppose that a subsidence of the ocean bottom should 
occur only in regions where the central islands of 
barrier reefs are present to attest it, and not in neigh- 
bouring regions where reefs of identical appearance, 
but without a central island, are given another name. 
The testimony of the second group of witnesses— 
massive elevated reefs such as occur on certain Fiji 
islands—convinced me that Darwin’s theory of subsi- 
dence gives the only satisfactory explanation of the 
origin of such reefs also; for their limestones rest 
unconformably on the normally eroded surface of a 
pre-existent foundation. The erosion of the founda- 
tion surface shows that it stood above sea-level before 
the reef was deposited upon it; and the occurrence of 
the reef shows that the eroded foundation subsided to 
NO. 2372, VOL. 95| 

| all around their shore line, and they are not. 

receive its marine cover. Only after this subsidence 
was the compound mass uplitted. The mere occur- 
rence of elevated reefs above sea-level does not for a 
moment prove that they were formed during the 
emergence of their foundation. 
All the still-stand theories of barrier reefs—that is, 
all the theories which involve a fixed relation of the 
reef foundation to the sea-level during the formation 
of the reef mass—are excluded by evidence of sub- 
mergence found in the embayed shore lines of the 
central islands within barrier reefs. It may seem 
over-bold thus at a stroke to set aside several well- 
known theories, accepted by experienced observers; 
and so indeed it would be if these observers had dis- 
cussed the features of the embayed central islands, 
and had explicitly shown that their embayments are 
not due to submergence but to some other cause. It 
is, however, a regrettable fact that the observers who 
adopted one or another of the still-stand theories 
took, like Darwin himself, practically no account of 
the embayed central islands, essential as the testimony 
of these islands is in the solution of the coral-reef 
problem. Such neglect is all the more remarkable 
in view of the clear statement, long ago published by 
' Dana, regarding the pertinence and the value of the 
testimony afforded by the central islands of barrier 
reefs. 
The glacial-control theory of coral reefs, recently 
elaborated by Daly with special reference to the 
lagoons of atolls, will not hold for barrier reefs. This 
theory assumes that no subsidence of the reef-founda- 
tions took place, and explains the lagoon floors of 
atolls as platforms abraded across pre-glacial sea-level 
reef-masses by the lowered and chilled sea of the 
glacial period after the corals were killed; the pre- 
glacial reef-masses having been formed by upward or 
outward growth on still-standing foundations. It 
then explains the encircling reefs which now surround 
the lagoons as having been built up while the sea was 
rising and warming in post-glacial time. But if the 
broad lagoons of large atolls twenty or thirty miles 
in diameter were thus formed, the central islands 
within narrow-lagoon barrier reefs should be cliffed 
Further- 
more, this theory explains the embayments of central 
islands within barrier reefs as occupying new-cut 
valleys that were eroded during the glacial period 
| of lowered sea-level; but if this were the case, the 
new-cut valleys should be prolonged upstream from 
the embayment heads as incisions in the floors of 
pre-glacial valleys, thus producing a ‘‘ valley-in-valley ”’ 
landscape; and this is not true in any one of the 
hundreds of embayments seen during the past year. 
Furthermore, many of the embayments are so wide 
that, if they were opened by slow subaerial processes, 
the spur-ends ought to have been well cliffed by the 
sea; yet, as above stated, they are not cliffed. Finally, 
many of the embayments are too wide to have been 
eroded during the last glacial epoch, or even during 
all the glacial epochs of the entire glacial period, if 
the valleys of the formerly glaciated volcanoes in 
central France are taken as standards of the amount 
of erosion that could be accomplished on such masses 
during such intervals of time. The glacial control 
theory thus proves incompetent to explain barrier 
reefs, and it is therefore held to be generally incom- 
petent to explain atolls also; it may have more im- 
portance on the borders of the coral zone, where the 
corals would most lilkkely have been killed during the 
glacial period: the Marquesas islands promise in- 
teresting results in this connection. The glacial- 
control theory has its greatest importance in conjunc- 
tion with Darwin’s theory of subsidence; for sub- 
