90 THREE CRUISES OF THE "'• BLAKE." 



the channels ; and that we can find nowhere any trace of eleva- 

 tion. Here the calcareous material has evidently been heaped 

 up to its highest point by the influence of the waves or winds. 

 Furthermore, we see growing to the westward of the Tortugas 

 a knoll similar to that which has formed the Tortugas them- 

 selves, and which will form, in the course of time, an island or 

 a series of islands like them to the westward. It is further 

 evident, also, that the Alacran Reef has been built up in the 

 same way, and that its peculiar atoll shape is due to the action 

 of the prevailing winds and currents, and not to any subsidence 

 of the great Yucatan plateau. 



The character of the fauna and flora of the Tortugas is inter- 

 esting as corroborating the comparatively recent age at which 

 the reef has been formed. We find, as we go north along the 

 keys, that the nearer we come to the mainland of Florida, the 

 greater the number of plants characteristic of the mainland. 

 As we reach islands more or less inaccessible, or islands merely 

 formed by flats which have reached low-water mark, the vege- 

 tation consists almost wholly of mangroves. Yet at the Tor- 

 tugas, in spite of the narrow channel which separates them 

 from the Marquesas, I saw but a single diminutive mangrove 

 plant, while a few bay-cedars, as they are called, a vine with a 

 thick white flower, and Bermuda grass have alone found their 

 way there, although the Tortugas are in the direct line of the 

 prevailing winds from the Marquesas. One of the species of 

 land shells common at Key West has already found its way 

 to the Tortugas. The group is visited by pelicans, cranes, 

 humming-birds, plovers, and a few land birds. It being the 

 winter season when I visited the place, the insects were few in 

 number. No terrestrial reptiles have been found on the Tor- 

 tugas, while at Key West there are many of the frogs, toads, 

 lizards, and snakes characteristic of the southern spit of the 

 mainland, — all this showing that the Tortugas reefs have not 

 been above the level of the sea long enough to have received 

 as yet the fauna or flora characteristic of the more northern fine 

 of keys. 



The explanation given here of the formation of huge deposits 

 of limestone from the limestone carcasses of invertebrates takes 



