50 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Boca Grande is in a condition which we may describe as a pre-atoll 

 stage. On the sea face of Boca Grande there is a steep bank thrown up 

 in front of a mangrove swamp occupying the southern part of the island. 

 At the base of the bank a broad expanse of beach rock is exposed, dip- 

 ping at a slight angle to the sea (Plate II.). The disintegration of the 

 beach rock has supplied the material for the coral sand bank with its 

 wide platform, which has been thrown up partly by the waves and 

 partly by the winds. It is easy to imagine this bank broken through, 

 with the sea making an irruption into the mangrove swamp and being 

 transformed to an irregular atoll, but iu fact a sound on a diminutive 

 scale. 



We find this to be the condition of Ballast Key, which represents an 

 atoll stage. The coral sand bank has been broken through, and a strong 

 current runs in and out of the gaps in the bank, and dunes have been 

 formed on the weather side of the Key. The material for the inner 

 dunes has been derived, as in the case of the Marquesas, from the 

 patches and stretches of beach rock flanking the outer shore line of the 

 coral sand beach. The outer shore line is composed of small man- 

 grove beaches, interrupted by clusters of large mangroves. Here, as at 

 Boca Grande, small mangroves are sprouting in the cracks and crevices 

 of the beach rock. (See Plate II.) The bottom on the sea face is 

 covered with similar blocks in all possible stages of disintegration. The 

 same is the case elsewhere along the keys, on the shores of the sounds, 

 and on the outer and inner reef flats. In a few feet of water masses of 

 gorgonians and of sponges are growing profusely, and extend into deep 

 water. 



From this my last examination of the Florida Keys I am inclined to 

 look upon the main line of the keys and of the patches of the outer 

 reef as the remnants of a long and wide belt of stretches and patches 

 of an elevated coral reef, which extended in more or less disconnected 

 patches at the Tortugas, at the ]\Iarquesas, and from the Marquesas 

 passage eastward to Key Biscayne, and even farther north, according to 

 Shaler, as a submerged reef perhaps as far as Jupiter Inlet. The keys 

 are all built upon this elevated coral reef foundation, which crops to the 

 surface, as we have seen, at many points, and from the beaches on the 

 sea face of this elevated reef has been obtained the oolitic material which 

 as seolian sand has raised the keys to a height of sometimes ten to 

 eighteen feet. This sand has hern blown to the northward, and filled 

 the sinks and sounds and channels separating the stretches of ri*r\\ and 

 extended a considerable distance inland, to form low ccolian hills and 



