STRUCTURE OF CORAL ISLANDS. 185 
on examination to afford examples of this variety of coral- 
rock. Such situations are identical with those on Oahu, 
where they occur on so remarkable a scale. 
In the Atlantic, on the Bermudas and Bahamas, and Flor- 
ida reefs, the drift-sand accumulations often have heights of 
forty to one hundred feet and sometimes of one hundred and 
fifty to two hundred and fifty feet. They make the dry land 
or islands and cays of these regions as described beyond. 
Although in these descriptions of atolls, some points have 
been dwelt upon more at length than in the description of 
barrier reefs, still it will be observed that the former have no 
essential peculiarities of structure apart from such as necessar- 
ily arise from the absence of high rocky lands. The encircling 
atoll reef corresponds with the outer reefs that enclose hich 
islands; and the green islands and the beach formations, in the 
two cases, originate in the same manner. 
The lagoons, moreover, are similar in character and posi- 
tion to the inner channels within barrier reefs; they receive 
coral material only from the action of degrading agents, be- 
cause no other source of detritus but the reefs is at hand. The 
accumulations going on within them are, therefore, wholly of 
coral. ‘The reefs within the lagoons correspond very exactly 
in mode of growth and other characters to the nner reefs un- 
der the lee of a barrier. 
IV. NOTICES OF SOME CORAL ISLANDS. 
The preceding descriptions represent the general character 
of atolls, but are more especially drawn from the Paumotus. 
There are some peculiarities in other seas, to which we may 
briefly allude. 
Among the scattered coral islands north of the Samoan 
