CORAL REEFS AND SUBMARINE BANKS 221 
rising and warming of the postglacial ocean, a reef, R, is built up 
on the platform margin, inclosing a lagoon, the inner shore line 
of which will [if no preglacial cliffs were cut, and if the low cliff, C, 
abraded during the glacial period, is submerged] be characterized 
by tapering spur-end points, PU, between embayed valley mouths, 
UY. The present depth of the lagoon, decreased by postglacial 
aggradation (vertical lines), must be less than the rock-bottom 
depth of 40 fathoms. 
In assuming that the spur ends will not be clift, this outline 
seems to me erroneous for two reasons: first, because the time 
required to reduce large preglacial islands to the broad platforms 
that are assumed to underlie the floors of large atolls and submarine 
Fic. 4.—Structure of a barrier reef, showing conditions which will result in 
producing a wide-lagoon barrier reef in postglacial time. 
banks, as in Fig. 2, ought to suffice for the cutting of strong spur-end | 
cliffs, like AN, Fig. 36, around the shores of younger and higher 
islands, particularly around such islands as today have narrow- 
lagoon barrier reefs, or only fringing reefs, like Rarotonga; secondly, 
because the time required to widen the deepened valleys, GC, by 
the slow process of weathering the resistant lavas on their sides so 
that they shall form embayments with the observed width of half 
a mile or more when the ocean resumes its normal level, ought 
surely to suffice for the cutting of strong spur-end cliffs. Whether 
these cliffs, AN, should be cut back of the inferred preglacial cliff, 
LF, is an uncertain matter; if so, the spurs today should terminate 
in steep and well-defined cliff faces, but this is rarely the case, as 
will be further shown in a later section; if not, a shallow rock plat- 
form, L, should front the weathered and battered preglacial cliffs, 
LF; but platforms so situated are unknown. 
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