218 W. M. DAVIS 
formed around it of detrital deposits, T’, in which coral reefs may 
be included. The ocean is then lowered about 40 fathoms from S, 
to S, during the glacial epochs of the glacial period, and the deeply 
weathered island is reduced by abrasion to a platform, B’XZ. 
The exposition of the glacial-control theory does not attempt to 
analyze the effects of alternating glacial and interglacial epochs, 
but assumes an almost continual action of abrasion, as is implied in 
the statement, ‘“‘The sea was actively attacking the islands and 
continental coasts throughout nearly the whole glacial period”’ (180). 
Such continuity of abrasion seems doubtful, in view of the greater 
length and probably higher temperature of the last interglacial 
epoch than of postglacial time; it is also questionable whether 
Fic. 2.—Structure of an atoll 
abrasion would be accomplished at the same level during successive 
epochs of glaciation; but I shall not pursue these details as the main 
question is sufficiently complicated without them. 
Let it be provisionally agreed, therefore, that abrasion was 
essentially continuous at one level; then the completely abraded 
platform must gain the form of a flat cone, the exterior part of which 
consists of a new ring of detritus, T’’. If a central remnant of a 
large island remains unconsumed, it should be rimmed with cliffs; 
but the residual granitic island of Mahé in the center of the vast 
Seychelles bank in the southern Indian Ocean is not cliff rimmed. 
If a preglacial island, 50 miles in diameter, be completely abraded, 
the platform may well be 20 fathoms deeper at its margin, B”, 
than at its center, Z. When the ocean warms and rises in post- 
glacial time and a reef, R, grows up on the platform margin, the 
exterior detrital slope, 7’’’, is slightly extended, and the lagoon is 
aggraded sufficiently (vertical lines) by the addition of horizontally 
stratified detritus, deposited unconformably upon the truncated 
volcanic rocks, to change the platform from a flat cone to a shallow 
saucer, ROO; or if, for any reason, the reef is not built up, the 
