FLORIDA BANKS. 207 
sand rock, and much of it into true odlite. The manner in 
which this consolidation is produced is explained on page 153. 
Pourtalés was the first to point out that the sand-made 
reef off the coast of Northern Florida continues to consist of 
siliceous sands quite down to Cape Floridi; that abruptly at 
this pot it becomes a coral-made reef, and is so continued \ 
southward. The facts mdicate, as observed by Major E. ice 
Hunt in 1863, that the drifting action of currents parallel 
with the coast have had much to do in locating the coral- 
built reef as well as the banks of siliceous sands; the 
material supplied is the chief difference between the two, 
coral sands failmg where growing corals fail. Major Hunt 
pointed out also that the current along the Florida coral reefs 
was a current counter to the Gulf Stream. cape 
How far the encroachment of siliceous sediments from the _ 
north may have determined the limit of coral-growing in that 
direction has not been ascertained. / If, as the author states 
in his Wilkes Expedition Report on Crustacea, the isocryme of 
68° F. terminates at Cape Canaveral, it would appear that the 
coral seas, or those warm enough to grow corals, extend one 
hundred and seventy-five miles north of Cape Florida, and that 
the northern siliceous sands have encroached all this distance. 
The limit may have been even north of Cape Canaveral ; 
for, as the author has recently learned from Professor Verrill, 
corals of the genus Oculina have been dredged up from a 
rocky bottom off Charleston, South Carolina, in depths of 
eight to ten fathoms. There is a question, however, whether 
the Oculina was one of the reef-making species. 
Little is seen over the flats or reefs of the true coral-reef 
rock, or the under-water coral limestone, which is the main 
material of all coral formations. It is generally concealed 
beneath the drift-sands of the surface. Tuomey observes 
