SUBMARINE DEPOSITS. 285 



want of light, etc., seemed to militate against this supposition. But 

 that animals living near the surface contribute also a not inconsider- 

 able proportion, is proved by the numerous shells of pteropods, occa- 

 sional teeth of fishes," etc. 



It was on this bottom that Pourtales dredged the interesting 

 little stalked erinoid Rhizocrinus, which he referred at first to 

 Bourgueticrinus. The importance attached to the subsequent 

 discovery of the identity of Bourgueticrinus with the Rhizo- 

 crinus dredged by Sars off the coast of Norway, as well as 

 by Pourtales, Agassiz, Thomson, and Carpenter, undoubtedly 

 awakened the general interest of naturalists to the problems of 

 thalassography, and to the two successful hauls of the dredge 

 in the Straits of Florida and off the Lofoten Islands we owe in 

 great part our recent deep-sea explorations. As stated by Pour- 

 tales, — 



" The sandy bottom ends exactly at Cape Florida. Key Biscayne, of 

 which the southern point is Cape Florida, consists in great part of 

 siliceous sand. The next island to the southward, only five miles dis- 

 tant, shows no trace of sand, but consists exclusively of coral lime- 

 stone, of which the whole range of the Florida Keys is also formed. 



" About Cape Sable siliceous sand reappears, and extends along the 

 western coast of Florida, though at first strongly mixed with lime. 



"It is remarkable how the littoral fauna changes with the constitu- 

 tion of the bottom. Many forms of animals peculiar to the Caroli- 

 nian favma disappear at Cape Florida, and reappear at Cape Sable and 

 on the west coast of the peninsula. Between these points they are en- 

 tirely crowded out by the interposition of the West Indian fauna of the 

 coral reefs. To take but one example, oysters are not found on the 

 coral bottom, though abundant to the east and west on the sandy 

 bottom." 



This shows that the nature of the bottom far more than the 

 depth is the cause of the abrupt changes of fauna which are 

 observed when we pass from one kind of bottom to another. 



To the westward of the shores of the southern extremity of 

 Florida we gradually come upon the belt of mud, more or less 

 mixed with calcareous matter, which stretches from Florida 

 towards deep water and around the shores of the Gulf of Mex- 

 ico, until it strikes again the calcareous deposits of the Yucatan 



