CORAL REEFS AND SUBMARINE BANKS 203 
probable explanation.* The lesson to be learned here is that the 
time which sufficed for the erosion of the steep-sided valleys of 
Tahiti sufficed also for the cutting of its great cliffs; hence, far from 
proving the power of fresh lavas to resist the breakers, Tahiti proves 
that, resistant as its lavas may be—and they certainly look very 
resistant where ledges are exposed in the cliff faces and on the valley 
sides—the waves of the trade-wind sea can strongly abrade the 
coastal slope of a volcanic island in the same period of time that is 
needed for the erosion of submature or mature valleys; and that 
the spur-end cliffs thus formed may be of such height that they will 
still rise hundreds of feet above sea-level after the cliff base and 
the platform in front of it and the valley ends between the clift 
spur ends are submerged hundreds of feet below sea-level. It there- 
fore still seems to me reasonable to be guided by the conclusion 
elsewhere set forth that the general absence of spur-end cliffs in the 
central islands of close-set barrier reefs and especially in islands like 
Rarotonga, that have only a fringing reef, contradicts the assump- 
tion that reef-building corals were killed and that the islands that 
they normally protect were exposed to abrasion during the glacial 
period. 
In order to avoid misunderstanding, let it be added that many 
spur ends of reef-encircled islands are nipped off in low, freshly cut 
bluffs,-5 to 30 feet in height, and that these low bluffs are fronted by 
wave-swept rock platforms of small breadth visible at low tide; 
hence these bluffs and platforms must be ascribed to wave work of 
the lagoon waters now or recently in progress at present sea-level. 
Further, let it be noted in passing that the subsidence inferred for 
Hawaii must have taken place while its gigantic young cones were 
in process of formation by eruption; and that a subsidence of 
Tahiti during its eruptive growth is also suggested, but less defi- 
nitely, by its association with its more submerged neighbors. 
These two instances deserve consideration as antidotes to the pre- 
vailing idea that volcanic eruption is necessarily associated with 
upheaval. Itis eminently possible, if not probable, that the reverse 
relation may often obtain, and still more possible that subsidence 
may set in shortly after eruption ceases. 
t “Clift Islands in the Coral Seas,”’ Proc. Nat. Acad. Sci., II (1916), 284-88. 
