CORAL REEFS AND SUBMARINE BANKS 305 
building polypifers can exist is partly determined by the ex- 
tent of inclined surface which the currents of the sea and 
the recoiling waves have the power to keep free from sedi- 
ments” (84), thus foreshadowing a view that is now generally 
accepted. 
A good number of other observers have interpreted the gradual 
slope and the steep pitch outside of a reef in the same way. Thus 
we read in the “Challenger’’ report regarding the reef at Tahiti, 
‘““The whole of the space from the edge of the reef to a depth of 
35 fathoms was covered with a most luxuriant growth of corals, 
with the exception of one or two small spaces where there was 
white coral sand”; the steeper pitch to greater depths was covered 
by coral blocks, “‘ which have been torn away from the ledge between 
the edge of the reef and 35 fathoms during storms, or by overhanging 
masses which have fallen by their own weight. In this way a talus 
has been formed on which the corals living down to 35 fathoms have 
found a foundation on which to build further seawards, for this 
[upper] slope is the great growing surface of the reef.”* It may be 
noted that the depth here given for abundant living corals is 
unusually great, and that Agassiz nearly thirty years later found 
a smaller proportion of growing corals and a larger proportion of 
dead corals, coral fragments, and coral sand on the same slope; 
hence the population of the slope presumably varies in relation to 
the master-storms of decades and centuries. But the important 
matter to note here in the present connection is that the outer 
slope of the reef, like the reef itself and the prograded ‘“‘belt of low 
land at the foot of the mountains”’ of Tahiti, represents an adjust- 
ment of aggrading processes and aggraded forms with respect to 
present sea-level, by whatever changes the present relation of land 
and sea-level have been brought about. 
Gardiner, whose studies of reef slopes are both intensive and 
extensive, says of the Fiji reefs, ‘“The section outside all is nearly the 
same, a gentle slope to about 40 fathoms and then a sudden steep.’’ 
t Narrative of the Cruise of H.M.S. ‘Challenger,’ I, Part 2 (London, 1888), pp. 779, 
781. 
2**The Coral Reefs of Funafuti, Rotuma, and Fiji... . ,” Proc. Cambr. Phil. 
Soc., IX (1898), 417-503; see p. 445. 
