16 FLOEIDA 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 Avhich 
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-shelled 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 with 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 leef 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, where 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 Carj^ophyllia rising between 
them, and hei'e and there a few Porites furcata. Such flats are very soft. 
