BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
3oca Grande is in a condition which we may describe as a pre-atoll 
stage. Оп the sea face of Boca Grande there is a steep bank thrown up 
in front of a mangrove swamp occupying the southern part of tho 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 П.). 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 tho waves and 
partly by the winds. It is easy to imagine this bank broken through, 
with the sea making an irruption iuto the mangrove swamp and being 
transformed to an irregular atoll, but in 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. Тһе 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 IL.) Тһе bottom on the sea face is 
covered with similar blocks in all possible stages of disintegration. Тһе 
same is the case elsewhere along the keys, on the shores of the sounds, 
and on the outer and inner reef flats. Ina 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 inelined to 
look upon the main line of the keys and оҒ the patehes 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 Marquesas, 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, аб many points, and from the beaches on the 
sea face of this elevated reef has been obtained the odlitic material which 
as solian sand has raised the keys to a height of sometimes ten to 
eighteen feet. This sand has been blown to the northward, and filled 
the sinks and sounds and channels separating the stretches of reef, and 
extended a considerable distance inland, to form low wolian hills and 
