158 ORGANIC AGENCIES. 



forms tlie south shore was a reef. Upon this reef by the action of waves 

 was gradually formed a line of coral islands, which finally coalesced into 

 a continuous line of land, and by the filling up of the intervening ship- 

 channel was added to the peninsula, the ship-channel being converted 

 into the present Everglades. In the mean time another reef was formed 

 in the position of the present line of keys. This has already been con- 

 verted into a line of wave-formed islands, and its ship-channel into shoal 

 water and mud-flats. Eventually the peninsula will be extended to the 

 line of keys, and the shoal water (e') will become another Everglades 

 and the mangrove islands its hummocks. Already another reef has 

 been again formed outside the last, viz., the present living reef (a" a"), 

 and upon it the process of island-formation has commenced. This will 

 also be eventually converted into a line of keys, into a continuous line 

 of land, and be added in its turn to the peninsula. It is not probable 

 that another reef will be formed outside of this, for the bottom slopes 

 rapidly under the Gulf Stream, as seen in the section Fig. 128. In 

 this process each reef dies when another is formed beyond it, for the 

 water being protected by the outside reef becomes placid or lagoon 

 water, and the strong reef -building species no longer flourish. 



North of the line d d the evidence is of the same kind, but less 

 complete. True reef-rock, similar to that now forming on the reef, 

 has been found at various points as far north as St. Augustine, on 

 the eastern shore. The western shore and interior are less known. 

 Tuomey in 1850 traced the Eocene on the west side as far as Tampa,* 

 and Smith in 1880 even to the north shores of the Everglades.f 

 The heavily shaded part, therefore, gives the probable outline of the 

 peninsula at the end of the Tertiary. If, however, as asserted by Agassiz, 

 superficial patches of coral, of species identical with those still on the 

 reefs, are found over this region, there must have been at least a tem- 

 porary submergence during the Quaternary. 



Mangrove Islands. — Mangrove-trees co-operate in an interesting 

 manner with corals in the process of land-formation. These trees 

 form dense jungles on the low, muddy shores of tropical regions. 

 They are very abundant on the shores of Florida. They have the re- 

 markable power of throwing out aerial roots from their trunks and 

 branches, thus forming subordinate connections with the ground or 

 with the bottom of shallow water. From these may spring other 

 trunks, which throw out similar roots, etc. Thus an inextricable en- 

 tanglement of roots and branches continues to extend far beyond the 

 actual shore-line. These form a nidus for the detention of sediments, 

 and protect them from the action of waves ; and the shore-line thus 

 steadily advances. 



* American Journal of Science, vol. i, p. 390, 1850. f Ibid., vol. xxi, p. 2S2, 1881. 



