REEFS OF FLORIDA. 157 



with its reefs and keys, and Fig. 128 is a section along the line N S. 

 The southern coast (a a) is ridge, elevated twelve to fifteen feet above 

 the sea-level, within which is the Everglades (e) an extensive fresh- 

 water swamp only two or three feet above sea-level, and dotted over with 

 small islands called hummocks. . Between the southern coast (a a) and 

 the line of keys (a' a') the water (e') is very shallow, only navigable to 

 smallest fishing-craft, and dotted over with small low mangrove islands. 



Fig. 128.— Section of same along line NS. Letters indicate the same. The dotted lines show sup- 

 posed previous conditions. 



A considerable portion of this area, in fact, forms mud-flats at low tide. 

 Between the line of keys (a' a') and the living reef (a" a") there is a 

 ship-channel (e") five to six fathoms deep. Outside the reef (a" a") the 

 bottom slopes rapidly into the almost unfathomable abyss of the Gulf 

 Stream (0 8 8). 



General Process of Formation. — Now, Agassiz * has proved that not 

 only the living reef but the keys, the southern coast, and the peninsula, 

 certainly as far north as the north shore of the Everglades (d d), and 

 probably on the east side as far north as St. Augustine (d'), have been 

 formed by coral agency. The evidence of this important conclusion 

 is that the rock in all these parts is identical with the reef -rock already 

 described, and with what is even now forming under our eyes on the liv- 

 ing reef (a" a"). It is, moreover, almost certain that the peninsula of 

 Florida has been progressively elongated by the formation of successive 

 barrier reefs, one outside of the other, from the north toward the south, 

 and the successive filling up of the intervening ship-channels, probably 

 by coral debris from the reef and sediments from the mainland. 



History of Changes — The history of changes was as follows : There 

 was a time when the north shore of the Everglades (d d) was the south- 

 ern limit of the peninsula. At that time the ridge (a a) which now 



* Coast Survey Report for 1851, p. 145 et 



