CORAL REEFS AND SUBMARINE BANKS 207 
Reef C, with sea-level at S,;, on the left side of the diagram is 
a barrier of greater thickness as a result of greater subsidence; its 
lagoon is of greater breadth than that of reef A or B’, and the 
embayments of its central island are strongly developed. Detrital 
deltas unconformably fill the embayment heads, and fringing 
reefs grow unconformably around the intervening spur ends. 
Borabora in the Society Islands has a barrier reef essentially of 
this kind, in which the surviving central mountain is greatly 
reduced from its initial form, and in which wide embayments enter 
between outspread spurs; the amount of subaérial erosion that 
must have taken place below present sea-level in order to reduce 
an initial cone to the existing mountainous form of this island is, 
in my judgment, far greater, both in depth and in volume, than 
could have been accomplished during the glacial epochs of lowered 
sea-level; hence the submergence inferred for this island should be 
ascribed chiefly to subsidence. 
If a pause occurred during the subsidence by which reef C was 
formed, so that sea-level remained at S, for a considerable period, 
the reef would grow outward on its own talus—this process having 
been distinctly recognized by Darwin, though it is usually credited 
to Murray: at the same time the lagoon would be shoaled or 
filled and thus a narrow young reef would be converted into a 
mature reef plain. Yap, in the Caroline Islands of the North 
Pacific, has a reef plain one or two miles in width interrupted by 
transverse channels, but without a lagoon proper. 
Reef D is an almost-atoll, in which only two small central 
volcanic knobs remain above water in the center of a broad and 
relatively shallow lagoon. Truk (Hogoleu), in the Caroline Islands, 
and the Gambier Islands, southeast of the Paumotus, represent 
this stage. Maré, one of the Loyalty Islands, north of New 
Caledonia, was formerly an atoll in which a small volcanic knob 
was Just submerged in the lagoon waters; the atoll is now elevated 
so that the broad lagoon plain, about 20 miles in diameter with the 
low volcanic knob near its center, stands 200 feet above sea-level and 
the reef rim rises some 50 feet higher. The knob has a gentle slope 
and shows no signs of cliffs, hence it cannot be regarded as a 
