286 THREE CRUISES OF THE “ BLAKE." 
Bank. The structure of the limestone banks which form the 
greater part of the peninsula of Florida, of Yucatan, of Hon- 
duras, of the Bahamas, and of the extensive ancient and mod- 
ern coral reefs of Cuba and the greater and lesser Antilles, 
and. extend along the northern coast of South America, has 
been already described in the chapter on the Florida Reefs. I 
merely allude to their general position to point out the connec- 
tion of these immense deposits of limestone with the calcareous 
bottom of the deeper parts of the floor of the Caribbean and of 
the Gulf of Mexico. 
The distribution of the recent coral reefs is shown on the 
map. (Fig. 191.) Тһе coral bottom, or coralline sand, which 
owes its origin to their presence, extends but a short distance in 
depth ; the coral reefs proper being sharply limited in depth, as 
reef-building corals only flourish within very moderate depths. 
On the surface, the slope is made up of fragments of shells, of 
the tests of invertebrates, and of similar material more or less 
broken and worn, which reaches to a depth of about a hundred 
fathoms or more. But nowhere in the West Indian area do we 
meet with the steep slopes which have been described as charae- 
teristic of the coral islands of the Pacific. 
Outside of the reefs we pass into a layer of soft, calcare- 
ous ooze, whiter nearer the reefs, and becoming somewhat col- 
ored at a greater distance. The whole deposit “is an im- 
mense layer of chalk, to which the organie life developed on its 
surface is constantly adding; while nearer shore the faunæ of 
the littoral and deep-sea regions, with their numerous corals and 
shells, contribute to the formation of limestone of various char- 
acters, such as odlite, muschelkalk, coral rag, and. conglomerates 
from beds broken up and reconstructed." 
The explorations of Pourtalds along the Florida reefs devel- 
oped an extensive limestone plateau, to which the name of Pour- 
talés Plateau has been given. This rocky plateau, with a very 
moderate slope, begins a little to the westward of Sand Key, 
and stretches to the northward and eastward, until it reaches its 
maximum breadth, of about eighteen nautical miles, to the east- 
ward of Sombrero. It then diminishes in breadth, and finally 
ends between Carysfort Reef and Cape Florida, at the same 
