The whole of this southern portion of the peninsula lies on a modern, coral formation. The crescent-shaped ridge which forms the eastern and southern boundary of the Everglades, commences north of Key Biscayne Bay, and sweeps southwest to Cape Sable. From the same starting point, another broken crescent of coralline limestone, but many miles longer, extends to the Dry Tortugas, forming the Florida Keys. And beyond this again some five or six miles, making a third crescent, is the Florida Reef. Outside of the Reef, the bottom abruptly sinks to a depth of 800 or 900 fathoms. Between the Reef and the Keys is the ship channel, about 6 fathoms in depth; and between the Keys and the main land the water is very shallow, and covers broad flats of white calcareous mud. Between the coast-ridge and Lake Okee-chobee, the Keys, which are scattered through the Everglades, are disposed in similar crescentic forms, some seven regular concentric arcs having been observed. They are all formed of the same character of coral rock as the present Reef and Keys, and undoubtedly owe their existence to the same agency. Each of these crescents was at one time a reef, until the industrious coral animals built another reef further out in the water, when the older line was broken up by the waves into small islands. Thus, for countless thousands of years, has this work of construction been going on around the extremity of the tertiary back bone ridge which at first projected but a short distance into the waters.
What, it may be asked, has impressed this peculiar and unusual crescentic shape to the reefs? This is owing to the Gulf Stream. This ocean-river rushes eastward through the Straits of Florida at the rate of five or six miles an hour, yet it does not wash the reef. By some obscure law of motion, an eddy counter-current is produced, moving westward