16 FLORIDA REEFS. 



upon all keys, whatever be their height ; and, lastly, that the level of the 

 highest keys, such as Key Largo and Key West, marks the height above 

 the sea-level to which the severest hurricanes, like that of 1846, have been 

 known to drive the loose materials. 



These facts in their connection are especially important, as giving us the 

 means of settling the question about the upheaval, depression, or perma- 

 nence of level of the coral fields of Florida. It should be mentioned here 

 that the fossil remains of animals and plants are rare in coral rock. It will 

 be seen hereafter that the mode of formation and accumulation to which 

 the reefs and keys owe their origin would be unfavorable to the preserva- 

 tion of such remains. When they are found in the coral rock, it is chiefly 

 in the brecciform limestone, where they have been sheltered in some exca- 

 vation, and thus protected from complete attrition. Here and there small 

 species of thick-slielled univalves occurs in a tolerable state of preservation, 

 and we have found several bones of a large turtle in the coarse oolite near 

 Key West. All the remains so found are identical witli species now living 

 on the reef 



Let us now return from this digression to the consideration of the keys 

 themselves, under the different aspects which they present. Some have 

 very abrupt shores, and rise like narrow ridges, with ragged edges and with- 

 out a beach, from the deeper water. These were undoubtedly formed upon 

 the narrowest parts of the old reef Others are more spreading, have 

 a wide beach, and dip gradually under the sea, their submarine slopes being 

 covered with coral sand and mud. These must have been formed on the 

 broader parts of the reef, wdiere it slopes gently on both sides. In still 

 others, the shores have been rendered abrupt by the denudation of some of 

 their earlier deposits. Such denudations may have been filled again by 

 more recent deposits, thus giving to a formation of the present geological 

 era a diversity usually characteristic rather of unconformable deposits 

 belonging to different geological ages. The northernmost keys, which con- 

 verge toward the main-land, are covered by silicious sand. Their beaches 

 are of like character, and slope towards the Atlantic, while their mud flats 

 spread along the western shores. South of Cape Florida no more silicious 

 sand is to be seen, and even in the immediate vicinity of Cape Biscayne 

 there is a mud shoal, laid partly bare at low water, over which grow branch- 

 ing Millepora, with small tufts of Oculina and Caryophyllia rising between 

 them, and here and there a few Porites furcata. Such flats are very soft. 



