Narrative of Bahama Expedition. 115 



of sand and rock formed from the skeletons of animals of 

 various sorts., and also from the secretions of corallines. The 

 latter, indeed, are regarded by some authors, e. g., Prof. 

 Louis Agassiz, as furnishing the main material for some of the 

 larger Keys. Of even more interest than the Keys, at least 

 from the naturalist's standpoint, are the reefs which almost 

 encircle the whole Tortugas group. During a heavy gale the 

 breakers are seen to form an often interruped but still quite 

 distinct line around almost the entire horizon, giving one the 

 impression that the form is essentially that of an atoll. These 

 reefs, especiallv the one stretching to the south and cast of 

 Garden Key, were objects of repeated visits by parties from 

 our schooner, affording an opportunity to observe some of the 

 phenomena included in the ever interesting coral-reef problem. 



Although no new facts were discovered beyond those men- 

 tioned by the older and the younger Agassiz, we found it well 

 worth our while to see some of these facts for ourselves, as 

 they are presented in connection with one of the youngest 

 reefs of the whole system in process of forming an extension 

 of the peninsula of Florida. 



We were unable to examine the outer or southern face of 

 the reef, as the breakers came in with great force during the 

 whole of our stay, although at times it was apparently quite 

 calm. We enjoyed the experience, however, of wading 

 around on the top of the reef and seeing the manner in wdiich 

 the debris is constantly being thrown inward and broken into 

 finer and finer fragments, until the bottom some distance in 

 from the exposed face of the reef is covered with a fine sand 

 or mud with only occasional fragments of coral of any consid- 

 erable size. On these mud-fiats we found a few living corals 

 and hosts of serpent-stars, echini and moUusks. 



One of our very best collecting grounds at the Tortugas 

 was in the extensive shallows stretching out northward from 

 Bird Key. The amount of coral, especially the madrepores, 

 which we found around these islands was not so great as we 

 had anticipated, on account of a considerable portion having 

 recently been killed by exceptionallv low tides. 



