300 CORALS AND CORAL ISLANDS. 
The remark: “It is a common observation in atolls that 
the islets on the reefs are situated close to the lagoon shore;” 
such “ facts point out the removal of matter which is going 
on in the lagoons and lagoon channels,” I know nothing to 
sustain. The width of the shore-platform on the seaward 
side is always greater than that on the lagoon side; but the 
outside shore-platform has its width determined by tidal and 
wave action, and this action is powerful on the ocean side, 
and feeble on the lagoon side; it produces a high coarse 
beach on the outside as the inner limit of the platform, and 
a finer, lower and much more gently sloping beach on the 
inside. The amount of erosion is far greater, as it should be, 
on the side of the powerful agencies. 
g. The loss to the lagoon by abrasion and solution is re- 
duced toa minimum, nm the majority of atolls, by the absence 
of lagoon entrances, which leaves them with only concealed 
leakage passages for slow discharge. 
Nine tenths of atolls under six miles in length (or in 
longer diameter), half of those between six and twenty 
miles, and the majority of all atolls in the Pacific ocean, 
have no entrances to the lagoon a fathom deep; and the 
larger part of those included in each of these groups have 
no open entrances at all. 
For evidence on this subject, reference may be made to 
the Wilkes Expedition Hydrographic Atlas. This atlas con- 
tains maps of nearly sixty coral islands from the surveys of 
its officers, drawn on a large scale — one or two miles, rarely 
four, to the inch. 
Out of the number, nine, ranging from 13 to 3 English 
miles in the longer diameter of the reef, have no lagoon, but 
only a small depression in its place; two of these take in 
water at high tide, and the rest are dry. 
