296 CORALS AND CORAL ISLANDS. 
barriers reaching far away from any land, and for the posi- 
tions and indented coasts of the small included lands. Is it 
a sufficient explanation of the facts? 
c. The currents that influence the structure of reefs are: 
(1) the general movement or drift of the ocean, in some parts 
varying with seasonal variations in the winds; (2) the cur- 
rents connected with wave action and the inflowing tide over 
a shelving bottom; and (3) the currents during the ebb, flow- 
ing out of channels; together with (4) counter-currents. 
Each region must have its special study in order to mark out 
all the local effects that currents occasion. Such effects are 
produced whether a secular subsidence 1s in progress or not, 
and hence a particular review of the subject in this place is 
unnecessary. 
The shaping of the outside of the reef and the determina- 
tion of the width and level surface of the shore-platform are 
due chiefly to the tidal flow and the accompanying action of 
wind-waves as explained on preceding page. 
The current that accompanies the ebb is locally the strong- 
est. Owing to the great width of many barrier reefs and of 
the channels and harbors within them, the tide flows in over 
a wide region. At the turn in the tide the waters escape at 
first freely over the same wide region; but with a tide of 
but two or three feet. there is little fall before the reef — 
which lies at low tide level and a little above it — retards it 
by friction; and thus escape by the open entrances is in- 
creased in-amount and in rate of flow. The facts are the 
same in atolls where the lagoons have entrances.) 
1 The currents of the tropical Pacific Ocean are of very unequal rate in its dif- 
ferent parts, and very feeble in the Paumotu Archipelago and the Tahitian and Sa- 
moan regions. Captain Wilkes reports that in the cruise of the Expedition through 
the Paumotu Archipelago to Tahiti, a distance of a thousand miles, during a month 
from August 13 to September 13, 1839, the drift of the vessels was only seventeen 
miles; and that during fourteen days in the first half of October, between Tahiti 
