668 
GEOLOGY: R. A. DALY 
to stir sand at depths of 40 or more meters. However, the leeward 
part of each atoll and barrier reef is usually discontinuous, the gaps in 
the reef wall being deep enough and wide enough to permit the ready 
escape of water blown thither by the wind. Hence, return or compen- 
satory currents are not likely to be efficient in undoing the work of the 
direct, wind-driven waves and currents. 
Consulting the charts and meteorological records, one finds scores of 
typical atolls located in belts where the trade winds cause the dominant 
waves and currents. The floor of each of these lagoons is strikingly 
level and not higher, more aggraded, either to leeward or to windward 
of the lagoon center. An example is the best known of all atolls, Funa- 
futi, in the Ellice group of the Pacific (fig. 2). Funafuti has occasional 
strong winds from westerly quadrants, but records show the dominance 
of the trade winds. Since the main reef at Funafuti is continuous for 
long stretches on the leeward side, its lagoon should, according to the 
subsidence theory, be distinctly shallower there than on the windward 
side. The charted soundings, actually much more nimierous than 
those entered in figure 2, demonstrate that the floor of the lagoon is 
nearly level and that the variations in depth have no systematic re- 
lation to the trend of the trade wind. 
The lagoon floors of many other Pacific atolls, including those of the 
Marshall and Paumotu groups, show similar levelness. Most barrier 
lagoons of the Fiji and other archipelagoes in the trade-wind belts 
are also characterized by practically the same depths to windward 
and leeward of their respective central islands. Exceptional lagoons 
in the southwest Pacific have been shallowed or deepened, uniformly or 
differentially, by recent, local warpings of the earth's crust; in these 
instances the test cannot be appKed with conclusiveness. Nor is it 
easily applied to the Maldives and neighboring atolls of the Indian 
ocean, where the monsoons change direction, through an angle of 180°, 
every few months. 
Nevertheless, in spite of the quite special weather conditions required 
by the test, very many atoll and barrier lagoons are known to be favor- 
ably situated, and yet none shows systematic lack of symmetry in the 
submarine profiles crossing its lagoon from windward to leeward. So 
also are the cross-profiles of the windward and leeward reefs themselves 
respectively of the same quality. The windward reefs tend to be more 
developed, but their inner slopes are of nearly the same steepness as the 
inner slopes of the corresponding leeward reefs. 
In conclusion, observed facts do not seem to agree with a legitimate 
deduction from the subsidence theory of coral reefs. On the other 
