GEOLOGY: R. A. DALY 
667 
Incidentally a question, no : directly the subject of the present com- 
munication, arises. Is it possible to credit this explanation of all the 
hundreds of atoll lagoons, every one of which exhibits no trace of the 
hypothetical moat? The entire absence of moat concavities in the 
world's atolls and the correlated flatness of barrier lagoons can only 
mean prolonged crustal stability in the reef -charged areas. 
The agents most responsible for the transportation of sediment into 
the depths of the hypothetical moat must be the waves and currents 
set going in the lagoon itself. The normal atoll or barrier reef, which 
reaches nearly or quite to sea-level, almost perfectly protects the lagoon 
from the powerful waves and currents of the open ocean. Some water 
is thrown over into the lagoon when great rollers break on low parts of 
the reef, with the effect of forming currents in the lagoon; but such 
currents cannot be other than weak. Tidal currents may be strong in 
narrow ship channels; inside the reef they are weak. The advance of 
detrital shelves, so as to fill the hypothetical moat, must, then, be very 
largely attributed to the activity of waves and currents originating in 
the lagoon. Except possibly during the rare hurricanes, these agents 
are incompetent to brush sand over the general bottom at the depths 
commonly ruling in atoll and barrier lagoons. But, assuming (in 
accordance with the hypothesis) that the transporting agents have 
been competent to fill the moats, an important effect on the form of the 
lagoon floor should be expected; the moat might in time be nearly filled, 
but the lagoon floor should not he level. 
That feature ought to be found especially in lagoons situated in the 
trade-wind belts and not affected by monsoons or other strong winds 
from westerly quadrants. In these cases the lagoon waves and currents 
must set in a westerly direction, and chiefly in that direction sediment 
should be transported. Hence the hypothetical moat, like its existing 
lagoon remnant, should be filled at different rates on the windward and 
leeward sides respectively. According to the degree of enclosure of the 
lagoon by continuous reefs, the leeward part of its floor should now be 
higher or lower than the windward part. To use a technical expression, 
the lagoon area should be differentially aggraded. The same tendency 
ought to be seen in lagoons subject to variable winds, among which is a 
dominant system of winds blowing from one quarter of the compass. 
To this reasoning it might be objected that back-running currents 
would reverse the direction of transportation for bottom debris. *Bank- 
ing' of water against the leeward section of the main reef would natu- 
rally cause return currents, if the reef were there continuous. Even 
in that case the currents must be comparatively feeble and unable 
