THE TORTUGAS AXD FLORIDA REEFS. 115 



of the peninsula of Florida, the shore line deposits, such as the Coquina of St. 

 Augustine and the shelly beaches of Indian River, seem to indicate the presence 

 in deep water of a limestone deposit formed of the detritus of Mollusks, Annelids, 

 Starfishes, and Sea-urchins. There are but few corals, occasional patches of reef- 

 builders, but no extensive reefs ; but this is a difficult point to decide even at Key 

 West. Indeed, all along the line of the reefs it would be difficult to decide to-day 

 whether the reefs have been formed, like the Marquesas Keys, merely by the accu- 

 mulation of detrital matter driven to the westward and northward, or whether the 

 Mangrove Keys and the reefs really indicate the old lines of a reef similar to the one 

 now in full activity on the northern edge of the Gulf Stream, parallel to the main 

 line of keys. The absence of the more delicate shells from limestone in the forma- 

 tion of which they must nevertheless have shared, is explained by the solvent action 

 of carbonic acid upon them, and by the deposition of carbonate of lime as a cement 

 A comparison of the structure of Loggerhead Key at the Tortugas with that of the 

 Mangrove 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 (Plate XIL). Yet we still find on the southern, eastern, 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, could not 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 (Plates X., XI.). 



We must be careful to distinguish the line of islands running from the mouth of 

 the St. John to Cape Florida, parallel with the coast of Florida, from the line of islands 

 forming the Florida Keys. The latter seem at first glance to be the continuation of 

 the former ; but this is not the case, their mode of formation, as well as their geological 

 structure, being radically different. As has long been pointed out by H. D. Rogers, 

 the line of narrow islands to the eastward of Florida belongs 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 causes, 

 perhaps by the action of currents along the continental shores forming lines of deposit 

 of but little width, separated from the mainland by a shallow channel. Some of these 

 islands have been slightly elevated at a comparatively recent period. This is espe- 

 cially the case with the islands along the east coast of Florida,— Anastasia Island, and 

 those running south to Cape Florida, separating Indian River from the Atlantic Ocean. 

 In all these we find the so-called Coquina of St. Augustine raised from ten to twenty 



