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 westwardof 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 line 
of keys. 
The explanation given here of the formation of huge deposits 
of limestone from the limestone carcasses of invertebrates takes 
