FORMATION OF CORAL REEFS AND ISLANDS. 251 
till they bridge over the bottom below. But such an event 
is not likely to happen in any but narrow channels. 
In recapitulation, the existence of passages through reefs, 
and the character of the coral harbors, may be attributed to 
the following causes : 
1. The configuration and character of the submarine land ; 
—corals not growing where the depth exceeds certain limits, 
or where there is no firm rocky basement for the plantation. 
2. The direction and force of marine currents, with their 
transported detritus ;—these currents having their course 
largely modified, if not determined, as in other regions, by the 
features of the land, the form of the sea-bottom, and the posi- 
tions of the reefs, and being sometimes increased in force by 
the contributions of island streams, which add to the detritus 
and to the weight of accumulating waters. 
3. Harbors which receive fresh-water streams, or submarine 
springs of fresh-water, are more apt to be clear from sunken 
patches; and the same causes keep open shallow passages to 
the shores, where there are shore reefs. 
It should be remembered, that while the effects from fresh- 
water streams are so trifling around islands, they may be of 
very wide influence on the shores of the continents where the 
streams are large and deep, and transport much detritus. 
This point is illustrated beyond. 
UU. ATOLL REEFS, 
The remarks on the preceding pages, respecting reefs around 
other lands, apply equally to atoll reefs. There are usually 
currents flowing to leeward through the lagoon, and out, over 
or through the leeward reef, the waves with the rising tide 
dashing over the windward side, and keeping up a large sup- 
