210 W. M. DAVIS 
threefold structure of a large barrier reef. It is perhaps for this 
reason that corals in place are rather seldom seen in elevated reefs; 
certainly they should not be expected in the exterior talus slope of 
a recently uplifted reef. The steep-pitching beds of the exterior 
talus may rest on the less steeply inclined strata of volcanic detritus 
asin reefs A and C; or on the horizontal lagoon beds of a drowned 
reef as in reef B’; or exceptionally on an eroded volcanic slope as 
in reef B’’, where the contact should be strongly unconformable. 
The horizontal beds of the lagoon limestones must, on approach- 
ing the central island, overlie the earlier-formed fringing reefs of 
the spur ends and the detrital deltas of the embayments; and the 
fringing reefs and deltas must in all cases rest unconformably on 
the eroded slopes of their foundation. This.is an important 
structural consequence of Darwin’s theory which has been very 
generally overlooked; it is well supported by many facts. The 
inner margin of the lagoon deposits, including the fringing reefs 
and the deltas, must follow a sinuous line, because as subsidence 
progresses the dissected volcanic slope must necessarily, as Dana 
showed, but as Darwin did not understand, have an embayed 
shore.* 
All of these features should be found in elevated reefs when they 
are dissected sufficiently to expose their structure. Among them 
all none is more important than the unconformity of the lagoon 
deposits on their eroded foundation, although mention has seldom 
been made of this significant structural feature in studies of coral 
reefs; for if the foundation is a slope of subaérial erosion—not a 
platform and cliff of marine abrasion—it must have stood above 
sea-level to suffer erosion before it was submerged to permit the 
formation of the unconformable and now elevated reef; and if the 
sloping surface of contact suffered a greater volume of erosion 
before the reef was formed upon it than could have taken place 
during the glacial epochs of lowered sea-level, or if it exceeds 240 
feet in vertical measure, its submergence should not be ascribed 
wholly to the postglacial rise of the ocean, but at least in part to 
subsidence. <A vertical measure of more than 600 feet is represented 
See Dana’s ‘‘Confirmation of Darwin’s Theory of Coral Reefs,’ Amer. Jour. 
Sci., XXXV (1913), 173-88. 
