GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 297 



area. The evidence is not decisive as to there having been a slight 

 emergence, of 6 to 10 feet, since the great submergence. 



As Verrill has shown, the Bermuda limestone is composed not of 

 coral debris, except in a subordinate proportion, but is made up of 

 broken, more or less triturated, calcareous tests, largely of mollusks. 

 He designates the material as "shell sands." The Bermudas are, 

 therefore, inappropriately called "coral islands." The recent corals 

 are growing on a foundation of older lime rock, brought into its 

 present relation to sea level by submergence. 



In that the last dominant change in the position of its strand line 

 was by submergence, Bermuda accords with the Florida coast, the 

 Bahamas, Cuba, and most of the smaller West Indian islands. 



FLORIDA. 



Strand-line oscillation in Florida has attracted the attention of 

 many geologists, among whom may be mentioned Shaler, Heilprin, 

 and Dall of the earlier investigators, and Matson, Sanford, Sellards, 

 Shaw, and myself of the later ones. . Shaw and I have recently 

 reviewed the subject. 1 That subsequent to formation of the Pleis- 

 tocene barrier reef of Florida, the reef tract was elevated to a height 

 about 50 feet above its previous stand and that this elevation was 

 followed by submergence to an amount of about 30 feet is shown by 

 (1) a submerged cave at Miami; (2) submerged solution well below 

 sea level, near East Bahia Honda Key; 2 (3) submerged peat bed at 

 Key West; (4) submerged indurated, cemented, recrystallized oolite 

 under the Marquesas; (5) submerged wave-cut terrace front at 

 Tortugas. 



In addition to this evidence Shaw and I say in the paper cited : 



Additional deductions of importance may be made from the submarine physiography 

 at depths beyond 10 fathoms. Although the investigations are at presept only in a 

 preliminary stage, it may be said that along the sides of the Gulf Stream from opposite 

 Miami to Satan and Vestal Shoals, just west of Sand Key, the Coast and Geodetic Survey 

 charts indicate fairly uniform slopes from 10 to 100 fathoms, but there may be narrow 

 terraces which are not brought out by the soundings. West of Vestal Shoal the sea 

 bottom drops suddenly from 10 to 20 fathoms, with a flat or gently sloping surface 

 between 21 and 28 fathoms. South of Goalbin Rock there is an escarpment between 

 10 and 30 fathoms, a flat or gentle slope between 30 and 40 fathoms, and another flat 

 or gently sloping area between 40 and 50 fathoms. The soundings are not sufficiently 

 numerous to trace surfaces with a feeling of confidence, but the scarp from 10 to 

 between 25 and 30 fathoms is clear cut and can be followed for 25 miles to the west end 

 of the Quicksands. Westward in the vicinity of Tortugas there are, besides, the 

 bottom of Tortugas lagoon and the surface of the shoal 7 to 10 miles west of Loggerhead 

 Key, two undersea terrace plains, one at a depth of 15 to 17 fathoms, the other, which 

 is a large plain west of Tortugas, ranges in depth from 28 fathoms on its landward to 

 36 fathoms on its seaward edge, and has an east and west width of 10 miles. The 15 

 to 18 fathom flat is especially well developed south and southwest of Tortugas. It is 



1 Vailghan, T. W., and Shaw, E. W., Geologic investigations of the Florida coral-reef tract, Carnegie 

 Inst. Washington Yearbook No. 14, pp. 232-238, 1916. 



2 Oral communication of Mr. Samuel Sanford. 



