72 THREE CRUISES OF THE “ BLAKE.” 
interesting reef as a sort of epitome of the mode of formation 
of the great Florida Reef and of the Bahamas Bank. Taken 
in connection with its position on the Yucatan Bank, it will 
perhaps explain the mode of formation of the greater part of 
the Florida peninsula as connected with the bank lying to the 
westward of the mainland and to the northward of the Florida 
reefs. 
The specimens thus far obtained from the rock composing the 
Yucatan plateau consist of the same limestone as those of the 
great Florida Bank; and there are forming upon it, though to 
a much more limited extert, patches of reefs near the thirty and 
twenty fathom curve, such as the Shoals, the Triangles, Cay Are- 
nas, the Arcas, the Madagascar, English, Alacran, and the 
fringing reefs of the eastern edge of Yucatan, that extend along 
the Mosquito coast to the central part of Central America. 
Tt is evident from the above that we need not refer the atoll- 
shaped form of this reef (Fig. 44) to the subsidence of the Yu- 
catan Bank as a whole, since the action of the prevailing winds 
and currents would account for all the existing phenomena. 
The decay of the animals living upon the great plateau, added 
to the deposition of all the animal life brought to it by the cur- 
rents, would explain a gradually increasing elevation of the sur- 
face till the level was reached at which reef-building corals 
could flourish, and at which a reef would naturally be formed. 
Darwin has noted the close resemblance between encircling bar- 
rier reefs and atolls. It seems to me that the structure of the 
Marquesas (Fig. 44) and of Alacran proves conclusively that 
not one point of difference exists between a barrier reef and an 
atoll. Darwin has also called attention to the fact that in 
shallow seas, such as the Persian Gulf and parts of the East 
Indian Archipelago, the reefs lose their fringing character and 
appear as irregularly scattered patches, often covering a con- 
siderable area; and he also observes that many reefs of the 
West Indies have been formed in like manner upon large and 
level banks lying a little beneath the surface, — banks which 
he believes to have been caused by the accumulation of sedi- 
ment. Such patches of reef-building corals would seem, from 
their analogy with the Tortugas, to be the beginning of more 
extensive reefs. 
