STRUCTURE OF CORAL REEFS. 143 
calcareous matter may be due to foraminifers, rather than cor- 
als, is not known.” 
Since the tidal waves on any coast that is gradually shal- 
lowing have a landward propelling power, the coral sands are 
mostly gathered about the reef, and generally are not to any 
great extent lost in the depths of the ocean. The great ocean- 
ic currents, like that of the Gulf stream, might bear away the 
lighter material for long distances, if it swept with full strength 
over the shore reefs; but it is generally true that such cur- 
rents are little felt close in shore. Notwithstanding the prox- 
imity of the Florida reets, and the strength of the Gulf stream 
in the channel between the Keys and Florida, the adjoining 
sea-bottom consists mainly of common inud, with relics of deep 
water life, and only sparingly of coral débris. According to 
Mr. L. F. de Pourtales, between twelve fathoms and one 
hundred, in the Florida channel, outside of the reef, coral frag- 
ments occur, but are rare; dead specimens of Cladocora and 
Oculina occur to a depth of about 50 fathoms. But on the 
other side of the channel, ‘‘along the Salt Key Bank, dead 
corals were dredged up in 315 fathoms ; but this is at the foot 
of a very steep slope washed by the edge of the Gulf stream ; 
which is much better defined here than on the Florida side.” 
The bottom, in the Florida channel, of 100 fathoms, is a rocky 
plateau, and outside of 200 fathoms, a mud full of foraminifers, 
Globigerina mud, as it is called from the species characterizing 
it; and yet this channel is situated beneath the Gulf stream and 
close by the Florida reefs. The facts seem to show that in most 
regions the reefs contribute little calcareous matter to the deep 
ocean. ‘This may be otherwise over the bottom, of compara- 
tively little depth, of a great Archipelago like that of the East 
Indies. 
