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 oilitic 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 parallel 
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 well 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 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 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 all these we 
find the so-called 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 shell débris 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 
