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 Tortuofas has larore heads of 

 astraeans (Fig. 54) and madrepores growing up on it at a depth 



Fig. 54. — Orbicella annularis. ( Agassiz. ) 



of from six to seven fathoms. Gorgoniae 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 "the 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." 



