304 W. M. DAVIS 
date, than with the uniform conditions demanded by the glacial- 
control theory. 
The exterior profile of coral reefs.—There is another feature of 
coral reefs to which Vaughan has called attention as indicating the 
existence of a submerged platform previous to, and independent 
of, the formation of the present reefs; this is the continuation of the 
shallow lagoon floor, not only where it is inclosed by a reef, but also 
through uninclosed sectors of its area where a marginal reef is 
wanting. A good number of examples of this kind are known in 
Fiji; but it is significant that the breach in the reef is in practically 
all cases on the leeward side. It is further significant that the 
uninclosed sector, which may like the rest of the lagoon floor have 
a general depth of 20 or 25 fathoms, slopes gradually to a depth 
of about 40 fathoms at its free margin and then pitches down with 
a steep descent to deep water; and it is still further significant that 
this change of declivity occurs at essentially the same depth as 
that at which the gentle exterior slope of the reef itself changes to a 
steep pitch. Instead of regarding these features as indicating the 
existence of a submerged platform of earlier and independent origin 
on which the reef was afterward built up, I am inclined to interpret 
both of them as resulting from aggradation by the transporting 
agencies of the ocean waters with respect to present ocean-level; 
in short, as small insular “shelves”? of rapid development with 
respect to the present level of the ocean and therefore as correspond- 
ing to great continental shelves of long-continued development with 
respect to the average relation of land and sea through modern 
geological periods. The change of slope outside of a continuous 
reef will be first considered. 
Darwin was, I believe, the first to suggest this view as to the 
origin of the exterior profile of coral reefs in his account of Keeling 
atoll. ‘As the external slope of the reef is the same round the whole 
of the atoll and round many other atolls, the angle of inclination 
must result from an adaptation between the growing powers of the 
coral and the force of the breakers and their action on the loose 
sediments”? (74); and he later added, ‘Considering the manner 
in which the beds of clean coral .... graduated into a sandy 
slope, it appears very probable that the depth at which reef- 
