for miles in length is changed from the submerged coral 
362 On Coral Reefs and Islands. te 
be inferred from the varied character of submarine slopes; in 
some cases the shallow waters may extend for two or three miles 
beyond the reef, but it is far more common to meet with the op- 
posite extreme—unfathomable depths within a few hundred feet. 
- ‘The growing corals are mostly confined to the shallow waters 
of the reef, and to its sloping margin up which they extend to 
within a foot or less of the surface. In these shallow waters the 
various zoophytes at times are crowded over extensive areas; yet 
very often they occur only in patches scattered throughout large 
fields of coral debris. The top of the reef is mostly destitute of 
life, and consists of the naked coral rock, more or less covere 
with coral sand. Yet there are some shallow pools, especially 
towards the outer limits, which abound in corals. 
The exposed edge of the reef is commonly raised a few inches 
above the general surface, and is, therefore, the first part laid bare 
by the retreating tide, although a dangerous place for a ramble, on 
account of the heavy breakers. Though very uneven, the surface 
has generally a smooth, water-worn appearance, and is spotted 
with various shades of pink and purple. These colors, as observ- 
ed by Chamisso, are due to incrusting Nullipores, that grow like 
lichens over the rock: they are vegetable in nature, though com- 
posed mostly of lime. Other nodular and branching Nullipores, — 
some sprigs of Madrepores, and a few of Astraeas grow in the more 
sheltered cavities, where they are not easily dislodged by the - 
waves; and among them, despite the breakers, cling numerous 
echini, asterias, and actiniw. The gradual wear of the reefs by 
the wash of the sea is prevented, to a great extent, by these Nul- 
lipore inerustations, as was pointed out by Darwin.* He states 
that on Keeling’s Island they constitute a layer two or three feet 
in thickness, with a breadth of twenty feet. ‘They are abundant 
on the Paumotu reefs. 
The outer reefs are distinguished in many parts from the inner 
by becoming covered with accumulations of coral fragments and 
sand, which are thrown up by the waves: finding a lodgment 
some distance back from the margin of the reef, the accumula- 
tious gradually increase, till in many instances they form dry land, 
‘and prepare the way for vegetation. Such effects are mostly con- 
fined, however, to the sides open to the prevailing wind, and are 
generally of limited extent. Occasionally, as at Bolabola, the po 
an 
into,a habitable islet-—a green belt to the island of rocks and for- 
f BP see ‘ ‘ * x 
ests within.. The causes and the result are much the same as in 
the case of the lagoon island, and the steps in the process will be 
-Thore particularly described when treating of the coral atoll. ° 
* Darwin,on Coral Reefs, London, 1842, page 9, and elsewhere. 
Pe sa 
