8 
ATOLLS. 
Ch. I. 
mark. I will describe the section, commencing with 
the outer margin. But I must first observe that the 
reef-building polypifers, not being tidal animals, require 
to be constantly submerged or washed by the breakers. 
I was assured by Mr. Liesk, an intelligent resident on 
these islands, as well as by some chiefs at Tahiti (Ota- 
No. 3. 
A — Level of the sea at low water : where the letter A is placed, 
the depth is 25 fathoms, and the distance rather more than 150 yards 
from the edge of the reef. 
B — Outer edge of that flat part of the reef, which dries at low 
water: the edge either consists of a convex mound, as represented, 
or of rugged points, like those a little farther seaward, beneath the 
water. 
C — A flat of coral-rock, covered at high water. 
D — A low projecting ledge of brecciated coral-rock, washed by the 
waves at high water. 
E — A slope of loose fragments, reached by the sea only during 
gales : the upper part, which is from six to twelve feet high, is 
clothed with vegetation. The surface of the islet gently slopes to 
the lagoon. 
F — Level of the lagoon at low water. 
neite) , that an exposure to the rays of the sun for a very 
short time invariably causes their destruction . 1 Hence 
1 [This would be true of certain genera or species, but according 
to the observations made during the voyage of the Challenger (Re- 
ports xvi. pp. 23, 36), and by Mr. Guppy (Proc. R. S. Edin. xiii. p. 863), 
there are some which can bear exposure for a limited time, perhaps 
two or three hours, especially if occasionally wetted by the spray.] 
