THE FLORIDA KEYS 41 



no traces of any of the large arboreal snails are to 

 be found. 



Geologists believe that the islands west of Key 

 West are of very recent origin, at least so far as 

 their elevation above the sea is concerned. While 

 those which lie between Key West and Boca 

 Grande are of oolitic formation the Marquesas 

 and Tortugas are composed of finely broken re- 

 mains of various marine animals. At Tortugas 

 extensive coral reefs have developed in the shal- 

 lows along the edges of the land, the finest growth 

 of this kind, perhaps, in the United States. The 

 Marquesas, which consist of one large and several 

 small islets, have an outline resembling a round- 

 headed kite. The group is really an atoll, the 

 outer keys forming a rim which encloses a shallow 

 lagoon. It is not, however, a true coral atoll like 

 those of the Pacific. The rock bed forming the 

 foundation of the Marquesas was probably built 

 up or elevated to very near the level of the sea. 

 Wave action afterwards heaped up sand around 

 the border and this now forms the dry land of. the 

 atoll. This sand covering has prevented or re- 

 tarded dissolution of the foundation rock, but in 

 the interior the less protected rock has been dis- 



