302 CORALS AND CORAL ISLANDS. 
by the theory to be most favorable in purity of water and in 
abundant life for progress; if the small cannot grow, the 
large lagoons cannot be made from them by the proposed 
method. 
Reverse the method, letting the large precede the small, 
and then, under the subsidence theory, we have a consistent 
order of events. We have large atoll reefs with several large 
entrances (like the great barrier reef about a high island in 
trances concurrently narrowing through the growing corals 
and the consolidating debris, in spite of the efforts of abra- 
sion and solution to keep them open and make them deeper ; 
and, afterwards, the atoll becoming still smaller until the 
entrances close up; and finally the lagoon-basin reduced to 
a dry depression with nothing of the old sea-water remaining 
except, perhaps, some of its gypsum. 
i. Instead of small lagoons having the purest waters, the 
reverse is most decidedly and manifestly the fact, and this 
accords with the reversal in the history just suggested. Since 
atolls of middle and larger size commonly have one third 
to two thirds of the encircling reef covered with the sea at 
one third tide, making the ocean and lagoon for more than 
half the time continuous, the large lagoon in such a case has 
as pure water as the ocean, and commonly as good a supply 
of food-life, and sometimes as brilliant a display of living 
corals. But in the smaller atolls, the area of the lagoon has 
little extent compared with the length and area of the encir- 
cling reef ; coral sands and other calcareous material conse- 
quently have possession of the larger part of the bottom, 
and the waters, since they are less pure than those outside, 
contain fewer and hardier kinds of corals and less life of 
other kinds. They are exposed, also, to wider variations of 
49 
this and other respects) gradually contracting, and the ont 
Aw 
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