THE FLORIDA REEFS. 67 



of carbonate of lime as a cement. A comparison of the struc- 

 ture of Loggerhead Key at the Tortugas with that of the Man- 

 grove Islands and the main keys shows us the difficulty of 

 deciding these points. At Loggerhead Key we have a shore 

 line made up of brecciated and oolitic coral limestone, fully as 

 characteristic as any similar shore line on the older keys like 

 Key West (Fig. 42.) Yet we still find on the southern, east- 

 ern, and northern sides of Loggerhead an active growth of 

 reef-building coral, while other parts of the island and some 

 of the flats, if covered by mangroves, cannot be distinguished 

 by their structure from the genuine mangrove islands on the 

 flats to the north of the inner line of keys along the main 

 reef. (Fig. 43.) 



We must be careful to distinguish the line of islands running 

 from the mouth of the St. John's to Cape Florida, and paraUel 

 with the coast of Florida, from the line of islands forming the 

 Florida Keys. The latter seem at first glance to be the con- 

 tinuation of the former ; but this is not the case, their mode of 

 formation, as weU as their geological structure, being radically 

 different. As was long since pointed out by H. D. Rogers, the 

 line of narrow islands to the eastward of Florida belonos to the 

 series of coast islands lying parallel to the coast from Long 

 Island to Florida, and extending around the whole Gulf of 

 Mexico. They all seem to have been formed by the same cause ; 

 the action of currents along the continental shores has formed 

 lines of deposit of but little width, and separated from the main- 

 land by a shallow channel. Some of these islands have been 

 slightly elevated at a comparatively recent period. This is es- 

 pecially the case with the islands along the east coast of Florida, 

 Anastasia Island, and those running south to Cape Florida, sep- 

 arating- Indian River from the Atlantic Ocean. In aU these we 

 find the so-caUed coquina of St. Augustine raised from ten to 

 twenty feet above the level of the sea at such points as Anas- 

 tasia, Merritt's, and Worth Islands, showing all along the east 

 shore of Florida a very recent formation of sheU debris or brec- 

 cia, very similar to the formation now going on in the lagoons 

 near Venice. This bed of shell breccia was probably deposited 

 after the low backbone of the peninsula, extending perhaps from 



