.s. ' 
STRUCTURE OF CORAL ISLANDS. 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 parailel 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 oc- 
currence of atolls or annular reefs within the larger atolls. 
yy Powells Is* 

MAHLOS MAHDOO ATOLL, WITH HORSBURGH ATOLL. 
Scale 1-20 of an inch to a mile. 
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. 
