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 reefs, and the strength of the Gulf stream 

 in the channel between the Keys and Florida, the adjoining 

 sea-bottom consists mainly of common mud, with relics of deep 

 .water life, and only sparingly of coral debris. 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 th'e Gulf stream ; 

 which is much better denned 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. 



