404 PRINCIPLES OF STRATIGRAPHY 



composed of one type of coral. Thus on the Organ Pipe Reef of 

 Thursday Island Tubipora musica predominates, while another reef 

 of this island is largely composed of the blue Heliopora cocrulea. 

 Other important contributors to the Great Barrier Reef material 

 are Euphyllia, Galaxea, Mussa, Trachyphyllia, Symphyllia, Gonias- 

 trsea, Prionastraea, Favia, Fungia, etc. ; none of these are, however, 

 as important as are the various species of Porites and Madrepora. 

 The hydrocorallines, IMillepora and the red Distichopora also add 

 materially to the lime of the great series of reefs. 



The reefs on the eastern coast of Africa are of the fringing 

 type, extending for a distance of nearly forty miles, from lat. i° 

 15' to 1° 45' S., and with an average distance from shore of some- 

 what more than a mile. The depth of the lagoon is very slight, and 

 this continues for some distance outside of the reef, where it is only 

 from 8 to 14 fathoms at a distance of a mile and a half from the 

 reef. The external margin of the reef is formed of projecting 

 points, within which there is a space from 6 to 12 feet deep with 

 patches of living coral on it. At Mombas (lat. 4° S.) a reef fringes 

 the coast for thirty-six miles, at a distance of from half a mile to 

 one mile and a quarter from the shore. The channel within this 

 reef, ranging in depth from 6 to 15 feet, is navigable for canoes 

 and small craft. Outside of the reef the depth is about 30 fathoms 

 at a distance of nearly half a mile. Part of this reef is very sym- 

 metrical, with a uniform breadth of 200 yards (Darwin-21). 



The reefs of the Florida coast form an interesting example of 

 the fringing type on a shallow continental platform. The southern 

 coast of Florida rises from 12 to 15 feet above the sea-level, in the 

 form of a curving ridge, which encloses an extensive fresh- water 

 swamp, the Everglades. The surface of this lies only two or three 

 feet above sea-level, and is dotted over with small islands, the so- 

 called hummocks. Some distance outside of the southern border 

 of the land lies a row of small islands or "keys" of dead coral 

 rock and sand, ranging from 5 to 30 miles distant from shore and 

 continued westward in a curved line far beyond the western coast 

 of the peninsula. The islands are low and of limited extent, that of 

 Key West near the western end of the line being less than 4 miles 

 long, while the longest of the islands is only 15 miles in length. 

 The keys slope toward their northern shore, and present a steep 

 face to the south, where they are separated from the living reefs 

 by an open channel. Between the keys and the southern coast of 

 the mainland the water is very shallow and navigable only to the 

 smallest boats. This lagoon is, moreover, dotted with small, low 

 mangrove islands. The mangrove trees growing here extend their 



