402 W. M. DAVIS 
agencies are at work on submarine banks, the fine detritus that they 
produce would be lifted from the shoal surface by the waves and 
currents and deposited on the steep exterior pitch; and the nearer 
the surface of the bank stood to the surface of the sea the more 
active would this process be. 
Evidently, then, if a former atoll were transformed into a 
submarine bank with a depth of 60 or 70 fathoms at a time of 
active subsidence, it might be aggraded to a depth of 50 or 4o 
fathoms during a following time of still-stand; but further reduction 
of depth might be long prevented by wave work if no coral rim 
were built up around the margin of the bank. Hence, if still-stand 
periods are commonly of long duration and times of active sub- 
sidence are short-lived, a rough approach to similarity in the depth 
of banks would ordinarily prevail; banks of unusually great depth 
would occur only here and there for a relatively short period after 
active subsidence and would therefore be exceptional. In the case 
of submarine banks that have not subsided from a former existence 
as islands, but that have been built up by volcanic eruptions to 
such depths as 200 or 100 fathoms and have then stood still, a similar 
balance might be reached when their depth was reduced by organic 
aggradation to the critical level. As was previously noted, what- 
ever truth there is in this supposition will militate against the 
acceptance of the Rein-Murray theory, which accounts for atolls 
by the progressive aggradation of still standing banks until they 
are brought to so small a depth that reef-building corals can be 
established upon them. 
Need of oceanic exploration.—It seems improbable, however, 
that subsidence, if it be really so dominant an oceanic process as 
Darwin’s theory implies, should not in some cases overcome 
aggradation and carry some banks down to such depths as 100 or 
200 fathoms. The scarcity of large flat banks at such depths is 
manifestly a difficulty that the theory of reef upgrowth does not 
fully overcome. It is of course possible that further exploration 
of the ocean depths will discover additional examples and show 
that they are not so rare as they now seem to be; and inasmuch as 
there are unsounded areas in the Pacific as large as Australia, this 
possibility may be almost a probability; nevertheless, as long as 
