88 THREE CRUISES OF THE ' BLAKE." 
eral thing parallel to the bend of the shore or at a slight angle 
from it, and dipping on the one side to the eastward and on the 
other to the westward. 
The bank to the west of the "Tortugas has large heads of 
astraans (Fig. 54) and madrepores growing up on it at а depth 
Fig. 54. — Orbicella annularis. (Agassiz. ) 
of from six to seven fathoms. Gorgoniw are still found at a 
somewhat greater depth. This bank is an excellent specimen 
of an isolated coral patch, such as must have formed the basis 
of all the keys and reefs. In time it will undoubtedly form a 
new reef, flush with the surface, to the westward of the Tortu- 
gas. The greatest depth at which reef-building corals have 
been observed to grow is in the Southwest Channel on the steep 
banks of White Shoal, and in the channel to the southwest 
of Bird Key, where madrepores grow to a depth of about ten 
fathoms! As a general rule, however, the corals are choked 
! Murray found off the slope of the covered with a most luxuriant growth of 
reef at Tahiti that е whole of the corals, with the exception of one or two 
space from the edge of the reef to a small spaces where there was white coral 
depth of about thirty-five fathoms was sand.” 
