MUSEUM OF COMPARATIVE ZOOLOGY. 135 



ence of a great atoll, Alacran Reef, on an area of elevation on huge 

 limestone banks such as those of Yucatan. 



Leconte insists on the fact that the Florida Reef, a true barrier reef, 

 has been formed where there could not be any subsidence, as continuous 

 increase of land is inconsistent with subsidence. According to Darwin, 

 barriers and atolls always show a loss of land, only a small portion of- 

 which is recovered by coral and wave agencies, while on the Florida coast, 

 according to Leconte and Agassiz, there has been a continuous growth 

 of the peninsula by coral accretion, until a very large area has been 

 added. 1 He attributed the formation of successive reefs to the suc- 

 cessive formation of the depth condition necessary for coral growth, 

 and this latter, in the absence of any evidence of elevation, to the 

 steady building up by sedimentary deposits and extension southward 

 of a submarine bank within the deep curve of the Gulf Stream as it 

 bent its way round the west coast of Florida. 2 The formation of bar- 

 rier reefs instead of fringing reefs on a coast which has certainly not 

 subsided, he attributes to the shallowness and muddiness of the bottom 

 along this coast. Only at a distance of twenty to forty miles, when the 

 depth of twenty-five fathoms is reached, and when, therefore, the bottom 

 is no longer changed by the waves, the conditions necessary for coral 

 growth could be found, and here a line of reefs would be formed, limited 

 on one side by the depth, on the other by the muddiness, of the water. 



According to Leconte the building up of Florida and of the Keys was 

 due to the co-operation of several agents : — 



1. The Gulf Stream building up and extending a submarine bank 

 within its loop, but not in the position assigned to it by Leconte. 



2. Corals building successive barriers on the bank, as the latter was 

 pushed farther and farther southward. 



3. Waves beating the reefs into islands. 



1 See Smith, Hilgard, Heilprin, and Dall, for the structure of the peninsula of 

 Florida. 



2 I can hardly see how Leconte states (Nature, October 4, 1880, p. 558) that 

 there are barrier reefs in Florida with lagoons from ten to forty miles wide, though 

 he subsequently (Nature, November 25, 1880) modifies this statement by indicating 

 this to mean the space between the southern coast of Florida and the line of Keys 

 /Old Barrier Reef) which widens from a few miles at its eastern part to more than 

 forty miles in its western part. But this is also misleading, as it refers to the time 

 when the Keys formed the reef, while now the channel between the line of Keys 

 and the present reef gradually widens from a narrow lagoon near Key Biscayne to 

 from six to ten miles wide, opposite the Marquesas, and is about one hundred and 

 fifty miles long. 



