CORAL REEFS AND SUBMARINE BANKS 307 
upgrowth of a narrow reef, or shoaled during stationary periods 
and periods of slow subsidence when the reef was broadened; and 
that the shoaling may have gone so far as to convert a narrow young 
reef and a deep lagoon into a mature reef plain,‘ if subsidence were 
long enough in abeyance. It is furthermore easily conceivable that 
prevalent subsidence may have been for a time neutralized in a 
phase of no submergence, while the ocean-level was falling as a 
glacial epoch came on, and then accelerated for a time in a phase 
of rapid submergence while the ocean-level was rising as a glacial 
epoch passed off; that a mature reef plain, formed during the 
neutralized phase of no submergence as a glacial epoch came on, 
might not be completely rimmed around by a new reef which grew 
up during the following phase of accelerated submergence as the 
glacial epoch passed off; and that in such case a failure of growth 
might reasonably enough take place on the border of the leeward 
sector of the reef plain, where the quantity of fine sediment shifted 
about by the waves might very likely prevent or retard coral growth. 
All of these mental schemes—mere figments of the imagination— 
are, as Is said above, easily conceived by anyone who cares deliber- 
ately to consider the coral-reef problem; the difficulty in the prob- 
lem lies elsewhere, namely, in the discovery of tests by which one 
of the schemes may be shown to represent better than any other 
the processes of the invisible past and thus to offer the best expla- 
nation for the invisible as well as the visible structures of the 
present. 
The best explanation that I have been able to reach is as follows. 
First, regarding the exterior profile of continuous reefs: In view 
of the evidence already given, from which it appears that the pro- 
duction of platforms by abrasion during the glacial period is improb- 
able if not impossible, and in view of the fair accordance between the 
depths at the outer margin of the uninclosed sector of a lagoon 
floor and the depth on the outer face of a reef where the change 
takes place from a gentle slope to a steep pitch, I am persuaded 
that the change of slope at 40 fathoms is not an inheritance from a 
time when that part of the reef lay at or close to the surface of the 
ocean, but, as already stated, a consequence of adjustment between 
t “The Great Barrier Reef of Australia,” Amer. Jour. Sci., XLIV (1917), 339-50; 
see p. 3406. 
