150 
THEORY OE THE FORMATION 
Ch. V. 
The Great Chagos Bank . 1 — I have already shown 
that the submerged condition of the Great Chagos 
bank (Plate II. fig. 1, with its section, fig. 2), and of 
some other banks in the Chagos group, may in all pro- 
bability be attributed to the corals having perished 
during an unusually rapid or sudden subsidence. The 
external rim or upper ledge (shaded in the chart) con- 
sists of dead coral-rock thinly covered with sand ; it 
lies at an average depth of between 5 and 8 fathoms, 
and perfectly resembles in form the annular reef of an 
atoll. The banks of the second level, the boundaries 
of which are marked by dotted lines in the chart, lie 
from about 15 to 20 fathoms beneath the surface ; 
they are several miles in breadth, and terminate in 
a very steep slope round the central expanse. This 
central expanse consists of a level muddy flat between 
80 and 40 fathoms deep. The banks of the second 
level appear at first sight to resemble the internal step- 
like ledges of dead coral-rock which border the lagoons 
of certain atolls, but their much greater width, and 
their being formed of sand, are points of essential dif- 
ference. On the eastern side of the atoll some of the 
banks are linear and parallel, like islets in a great river, 
and they point directly towards a great breach on the 
opposite side of the atoll : these are best seen in the 
large published chart. I inferred from this circum- 
stance, that strong currents sometimes set directly 
across this great bank ; and I hear from Captain Moresby 
that this is the case. I observed, also, that the channels, 
1 [See Appendix II.] 
