54 
ATOLLS. 
Ch. I. 
nary atolls, some annular reefs rising to the surface 
but without any islets on them, and some atoll- formed 
banks either quite or nearly submerged. Of the 
latter, the Great Chagos Bank is much the largest, and 
differs in its structure from the others ; a plan of it 
is given in Plate II. fig. 1, in which, for the sake of 
clearness, I have had the parts under ten fathoms deep 
finely shaded : an east and west vertical section is given 
in fig. 2, in which the vertical scale has been neces- 
sarily exaggerated. Its longest axis is ninety nautical 
miles, and another line drawn across the broadest part, 
at right angles to the first, is seventy. The central 
part consists of a level muddy flat between forty and 
fifty fathoms deep, which is surrounded on all sides, 
with the exception of some breaches, by the steep 
edges of a set of banks rudely arranged in a circle. 
These banks consist of sand with a very little live 
coral ; they vary in breadth from five to twelve miles, 
and on an average lie about sixteen fathoms beneath 
the surface ; they are bordered by the steep edges of a 
third narrow and upper bank, which forms the rim to 
the whole. This rim is about a mile in width, and, with 
the exception of two or three spots where islets have 
been formed, is submerged between five and ten fathoms. 
It consists of smooth hard rock, covered with a thin 
layer of sand, but with scarcely any live coral; it is 
steep on both sides, and slopes abruptly outwards into 
unfathomable depths. At the distance of less than half 
a mile from one part, no bottom was found with 190 
fathoms ; and off another point, at a somewhat greater 
