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 m 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.) The 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 sheUs, 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 charac- 

 teristic of the coral islands of the Pacific. 



Outside of the reefs Ave 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 organic life developed on its 

 surface is constantly adding ; while nearer shore the faunae 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 oolite, muschelkalk, coral rag, and conglomerates 

 from beds broken uj) and reconstructed." 



The explorations of Pourtales along the Florida reefs devel- 

 oped an extensive limestone plateau, to which the name of Pour- 

 tales 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 



