474 
GEOLOGY: W. M. DAVIS 
chilled enough to kill the reef-building corals, the island was progres- 
sively cut away by the sea, sectors C, D, and eventually reduced to a 
platform a little below the sea surface, sector E: when the sea rose and 
warmed in Postglacial time, a reef was built up around the platform 
margin, and the enclosed area was covered with lagoon deposits, the 
resulting structure being shown in section on the face of sector F. 
FIG. 2. 
Now if such an atoll, sector F, figure 3, be upKfted more than 40 
fathoms or 240 feet, as in sector G, and eroded, as in sector H, the 
central volcanic area of the platform will be sooner or later laid bare, 
as in sector J. It is evident, however, that no great amount of erosion 
can take place in Postglacial time ; hence it must here be assumed that 
uplifted atolls which are much dissected were uplifted during the 
FIG. 3. 
Glacial period, and then deeply dissected by subaerial erosion and 
benched by marginal abrasion during the last Glacial epoch of lowered 
sea level, so that when the sea finally rose in Postglacial time, an out- 
standing barrier reef would grow up around the margin of the last 
abraded marginal bench and enclose a lagoon, the waters of which would 
enter narrow embayments in the central island, as in sector H. In 
