292 W. M. DAVIS 
discussion, namely, that volcanic islands which are indented by 
drowned-valley embayments half a mile or a mile wide must, like 
northeastern Hawaii, have had strong cliffs cut in their spur ends 
if they were exposed to abrasion during the considerable period 
required for the erosion of the now drowned valleys; and as such 
cliffs are prevailingly absent on reef-encircled islands the islands 
must have been protected from abrasion—that is, their reefs must 
have been clothed with growing organisms of some sort—while 
the now drowned valleys were eroded. 
Tahiti gives even stronger evidence to the same end. This 
island consists of a larger and a smaller volcanic cone, joined in an 
isthmus. The slopes of the cones are submaturely or maturely 
dissected by deep, steep-sided, radial valleys, the lower ends of 
which have been embayed by submergence after they were eroded; 
but the embayments are now, with few exceptions, filled with 
deltas which have expanded outside of their embayments so as to 
form a continuous alluvial belt around much of the shore line; it 
was this “broad belt of low land at the foot of the mountains” 
which Darwin properly interpreted as indicating “‘a long stationary 
period” for Tahiti. It is, however, not the radial valleys but the 
spur-end cliffs that are the most significant feature of Tahiti in 
the present connection. Agassiz gave a good account of them. 
Some of the cliffs rise 500 or 1,000 feet above present sea-level on 
the more exposed coasts; but the northwestern or leeward corner 
of the island is without visible cliffs for a short distance. Their 
origin must have been contemporaneous with the erosion of the 
valleys; hence the base of the cliffs and the platform that must 
have been abraded at a small depth below sea-level when the cliffs 
were cut back must be now, like the distal portion of the valleys, 
submerged. The depth of submergence, inferred from the cross- 
section of some of the larger valleys, may well be 500 or 600 feet; 
the lagoon, much aggraded, is naturally of less depth. 
It is immaterial for the moment whether the opportunity for 
cliff-cutting around Tahiti was given while the ocean was chilled 
and lowered in the glacial period, or whether it was given while the 
island stood higher and was, like Reunion, reefless because a sheet of 
shore detritus prevented coral growth, as I believe is the more 
