GEOLOGY: W. M. DAVIS 
151 
Darwin's original theory of subsidence gives by far the most satis- 
factory explanation of all the barrier reefs that I have visited in the 
Pacific or studied on large-scale charts; and as atolls often occur in 
association with barrier reefs, the theory of subsidence appears to give 
the best explanation of such atolls also. Atolls that are not associated 
with barrier reefs may be of some other origin, but this does not seem 
probable. 
The elevated reef along the south coast of Oahu, Hawaii, was formed 
during or after a sub-recent period of subsidence, for its limestones 
enter well-defined valleys of erosion. 
The Fiji group has suffered various movements of subsidence and 
uplift by which its many islands were affected in unlike ways. Uplift 
has taken place at different times, for some of the elevated reefs are 
elaborately dissected, others are very little dissected, and still others 
remain at sea level. The upHfted reefs seem to rest unconformabi)' 
on subaerially eroded volcanic centers, hence the centers must have been 
above sea level to suffer erosion; they must have been depressed to 
receive the unconformable reef deposits; and the compound mass must 
then have been elevated to lay bare the reef. In one island (Vanua 
Mbalavu) the upKfted reef has been maturely dissected and partly 
submerged, as indicated by its embayed border, and a new barrier reef 
has grown up outside of it. Thus all the Fiji reefs, those now elevated 
as well as those at sea level, appear to have been formed during periods 
of subsidence. 
The extensive barrier reef of New Caledonia has grown up during a 
recent subsidence by which that long and maturely dissected island 
has been much reduced in size and elaborately embayed; but unlike 
most encircled islands this one was strongly cliffed around its south- 
eastern end and along much of its northeastern side before the recent 
subsidence took place. 
The two southeastern members. Mare and Lifu, of the Loyalty 
group, are former atolls, evenly uplifted 200 or 300 feet; Mare has a small 
knob of volcanic rock in its centre. Uvea, the northwestern of the 
three Loyalty islands, is a slightly tilted atoll. 
The New Hebrides show signs of uplifts in their elevated reefs, and of 
depressions in their embayments. There is some evidence that cer- 
tain uplifted fringing reefs on the island of Efate, near the center of the 
group, were formed during pauses in a subsidence that preceded their 
uplift, and not during pauses in their upHft as inferred by Mawson. 
Espiritu Santo, in the northwest, has several large embayments inter- 
