64 Papers from the Marine Biological Laboratory at Tortugas. 



the head of Florida Bay, with its salients, reentrants, and almost numberless 

 small islands lying near shore, as Ten Thousand Islands, separated by an 

 interlocking maze of channels, suggests that the southern end of the penin- 

 sula to its very tip has undergone subsidence at no distant date. On keys 

 west of Bahia Honda Sanford noted holes, which, at a distance of a quarter 

 of a mile or more from shore, connect with underground passages containing 

 salt water; and on more easterly keys formed of elevated coral reef rock he 

 observed free openings or cavities that extend to a depth as great as 30 feet 

 below sea-level. The evidence is clear that the keys participated in the 

 uplift and subsequent depression that affected the mainland and that at 

 one time they stood more than 30 feet higher with reference to sea-level 

 than they now do. This uplift and the subsequent depression, according 

 to all available evidence, extended to the Tortugas. 



The highest points to which the elevated reef extends are on Windleys 

 Island and Plantation Key, where elevations of 18 feet are attained. Sup- 

 posing these points to have been at sea-level, an uplift with reference to 

 sea-level, of somewhat more than 48 feet, must have taken place since the 

 elevated reef was formed. 



The evidence presented shows that the platform, on the outer edge of 

 which the present barrier reef of Florida is principally growing, has geo- 

 logically, just antecedent to its present relations to sea-level, stood 30 feet 

 or more higher. It is on this shelf, the last oscillatory movement of which 

 was one of depression, that the living reefs of the Florida barrier have 

 established themselves and have grown. However, although the last earth 

 movement has been downward, the net result of the last oscillation, i. e., 

 uplift followed by subsidence, has been to bring the surface of the sub" 

 marine shelf nearer to the surface of the sea. Although the building of the 

 platform seaward of the Pleistocene barrier reef needs further study, it is 

 known that the base of the Pleistocene reef is about 100 feet below sea- 

 level. As the reef would not have begun to grow in a marked depression, 

 it is probable that the water seaward of it was not shallower than that at 

 its base. The maximum depth of Hawk Channel is about 7 fathoms, while 

 in places the living barrier reaches the surface of the sea. Therefore, since 

 the initiation of the Pleistocene reef the bottom seaward of it has been 

 built up to a thickness ranging from 70 to 100 feet. As the question, how 

 much of this material may be Pleistocene, can not now be answered, it is 

 not known whether or not the living reef has Pleistocene corals beneath it. 

 However these questions may ultimately be answered, it is clear that both 

 addition of material (upbuilding) and uplift have contributed to raising 

 the surface of the basement of the Recent reef, notwithstanding that the 

 last oscillatory movement carried the land surface downward. The hydro- 

 graphic charts (see particularly U. S. Coast and Geodetic Chart No. 165, 

 Hillsboro Inlet and Fowey Rocks) clearly show the northward extension 

 of the reef platform beyond the limits of the living reefs, which are 

 merely superimposed on a platform whose existence antedates their own. 



