1836.] OR ATOLLS. 467 
plicable to the northern Maldiva atolls in the Indian Ocean 
(one of which is 88 miles in length, and between 10 and 20 in 
breadth), for they are not bounded like ordinary atolls by narrow 
reefs, but by a vast number of separate little atolls; other little 
atolls rising out of the great central Jagoon-like spaces. A third 
and better theory was advanced by Chamisso, who thought that 
from the corals growing more vigorously where exposed to the 
open sea, as undoubtedly is the case, the outer edges would grow 
up from the general foundation before any other part, and that 
this would account for the ring or cup-shaped structure. But 
we shall immediately see, that in this, as well as in the crater- 
theory, a most important consideration has been overlooked, 
namely, on what have the reef-building corals, which cannot live 
at a great depth, based their massive structures ? 
Numerous soundings were carefully taken by Captain Fitz 
Roy on the steep outside of Keeling atoll, and it was found that 
within ten fathoms, the prepared tallow at the bottom of the 
lead, invariably came up marked with the impressions of living 
corals, but as perfectly clean as if it had been dropped ona 
carpet of turf; as the depth increased, the impressions became 
less numerous, but the adhering particles of sand more and more 
numerous, until at last it was evident that the bottom consisted 
of a smooth sandy layer: to carry on the analogy of the turf, 
the blades of grass grew thinner and thinner, till at last the soil 
was so sterile, that nothing sprang from it. From these obser- 
vations, confirmed by many others, it may be safely inferred that 
the utmost depth at which corals can construct reefs is between 
20 and 30 fathoms. Now there are enormous areas in the 
Pacific and Indian Oceans, in which every single island is 
of coral formation, and is raised only to that height to which 
the waves can throw up fragments, and the winds pile up sand. 
Thus the Radack group of atolls is an irregular square, 520 
miles long and 240 broad ; the Low archipelago is elliptic-formed, 
840 miles in its longer, and 420 in its shorter axis: there are 
other small groups and single low islands between these two 
archipelagoes, making a linear space of ocean actually more than 
4000 miles in length, in which not one single island rises above 
the specified height. Again, in the Indian Ocean there isa 
space of ocean 1500 miles in length, including three archi- 
