4o6 PRINCIPLES OF STRATIGRAPHY 



clastic mud sediment rich in organic matter rests directly upon the 

 ancient coral reef now represented by the keys, a relationship ex- 

 pressed in the rocks of the older geological periods by the super- 

 position of a black carbonaceous shale above an earlier coral lime- 

 stone. 



Outside of the line of keys and from 3 to 15 miles distant from 

 it is a line of living coral reefs, consisting of mounds made up of 

 branching madrepores, Porites, etc., besides many smaller genera 

 such as Manicina, Agaricia, etc. Corallina and Lithothamnion also 

 add a large percentage of calcareous material to the reef. 



These reefs are for the most part submerged, rising only here 

 and there to the surface. Between them and the keys lies the outer 

 lagoon, a long, narrow channel five to six fathoms deep and naviga- 

 ble for small vessels. Here the sedimentation consists of coral 

 sand and of the shells of marine organisms, thus producing a nor- 

 mal marine limestone, which is either in the form of a coral breccia 

 or a more or less oolitic calcarenyte. Outside the living reef, the 

 bottom rapidly descends to the abyssal depths of the Florida Straits 

 (2,916 feet). 



While nullipores, or the stony algae, luxuriate in the outer zone 

 of the reef, where they form a distinct Nullipore zone in the face 

 of the strong surf, the more delicate branching and brittle corallines 

 are confined to the channels and lagoons within the reef, where 

 they often form thick carpets in the -quiet water. Thus the shallow 

 parts of the bottom of the ship channel between the living Florida 

 reefs and the old reefs or keys are covered with the so-called "coun- 

 try grass," one of these calcareous alga;. This is especially notice- 

 able between Fowey Rocks, Triumph Reef and Long Reef on the 

 one side, and Soldier Key and Ragged Keys on the inside (Agassiz- 

 2:126, 12'/). The floor of Hawk Channel, which has a depth of 

 from 6 to 7 fathoms, is covered with disintegrated corals and coral- 

 lines (Pourtales; Dana-2o:i'/i), while some of the keys in the Dry 

 Tortugas and Marquesas are wholly composed of fragments of 

 corallines bound together into a solid mass. Among these coral- 

 lines a large species of Opuntia is especially noticeable. 



It has been shown by Agassiz that the keys, the southern rim 

 of the mainland, and a strip including the north shore of the Ever- 

 glades and extending as far north on the eastern shore as St. Au- 

 gustine, are of coral-reef origin. When this last strip was the living 

 reef, the platform in front was being built out into the sea by the 

 accumulations of the shells of organisms which lived in abundance 

 in the genial waters of the Gulf Stream border. When the platform 

 was sufficiently extended a new line of reefs came into existence, 



