122 THE TORTUGAS AND FLORIDA REEFS. 



rents. The fact that the coral reef at the extremity of Florida is the most recent 

 of the coral formations found on the Florida shores, plainly shows that they, as well 

 as the coral reefs of Yucatan, Cuba, the Bahamas, and the Caribbean Sea, though 

 not all of the same age, were yet of modern origin, since we find them still in an 

 active state of formation. Even the elevated reefs of Cuba and of the other West 

 India Islands, though older, probably belong, nevertheless, to the most recent deposits 

 of the kind we know. The difficulty of explaining the constant renewal of the coral 

 faces of the atolls of the Pacific, and their present condition, on the supposition of 

 their having lived from the time of the early Tertiaries, was one of the main causes 

 which led Darwin to seek for some other agency, like subsidence, to explain the 

 renovating process of the original structure. In some instances coral reefs have 

 unquestionably been lifted. I have seen the elevated reefs of Cuba, of San Domingo, 

 and other "West India Islands, and especially of Barbados, 1 which are perhaps the most 

 striking of these. They are too well known to need more than a passing notice 

 here. The terraces they form show plainly the successive stages of arrest in the 



■ 



agency of elevation, and there is no difficulty in accounting for their existence, 

 especially in a volcanic region like the West Indies; but that there should have 

 been an extensive area of subsidence in which the rate of subsidence was so evenly 

 balanced with the rate of coral growth as to create and maintain the necessary con- 

 ditions for reef formation, is less easy of explanation. 



In a very interesting article on the Bermudas, Rein 2 has taken very much the 

 same view of their gradual building up, and explains the formation of the present 

 condition of things by causes greatly differing from those adduced by Darwin as 

 explaining the apparent atoll shape of the groups. . 



The islands composing the Tortugas (Plates I and II) are Loggerhead, Bird, Garden, 

 Long, Sand, Middle, and East Keys. These are always above the level of the sea, 

 while Southwest Key and Bush Key are only exposed at low water, and North Key 

 and Northeast Key have disappeared. These insignificant islands are the outcrops 

 of extensive submarine banks. Loggerhead Key, not more than three fourths of a 

 mile in length, is the top of a bank extending to the three-fathom line of about five 



lensrth with an average width of three fourths of a mile, extensive coral 



■O ~ " «»^»g 



sand-flats running in prolongation of the northern and southern extremities of the 



1 The trachytic cone forming the base upon which the successive terraces of Barbados hare been elevated is seen to crop 

 out on the surface in the northeastern part of the island. 



2 Rein, J. J. Beitrage zur Physikalischen Geographie der Bermuda Inseln. Bericht. Senckenb. Naturf. Gesell., 1869-70, 

 (Mai 1870,) pp. 140-158. 



