Corals and coral islands.

STR UCTURE OF CORAL ISLANDS.18

189

Royal Geographical Society, vol. v., p. 398; and another on
this group by J. J. Horsburgh, and W. F. W. Owen, in the
the same journal, vol. ii., pp. '72 and 81. As stated by Mr.
Darwin, the archipelago has a length of 470 miles, with an
average breadth of 50 miles; and it consists for the most of
its length of two parallel lines of atolls. The large atoll at
the north end has a length of 88 miles, while Suadiva, one of
the southernmost, is 44 miles long from north to south, and
34 miles across.
The point of special interest in their structure 'is the occurrence of atolls or annular reefs within the larger atolls.
Pow ell t sgi
0:
fr 0
J
z0
HorsburghAtoI1 -MAHLOS MAHDOO ATOLL, WITH HIORSBUR~GII ATOLL.
Scale 1-20 of an inch to amfZ&
The islets of the lagoon and those of the encircling reef, are
in many instances annular reefs, each with "its own little lake.
Gems within gems are here clustered together.