236 CARNEGIE INSTITUTION OF WASHINGTON. 



Mr. H. L. Cook, a contractor formerly with the Florida East Coast Railway, 

 reports that near Harris Key, north of the town of Chase, on Sugarloaf Key, 

 submerged peat was found 6, 14, and 18 feet thick, overlain by marl, and 

 underlain by solid rock. Submerged peat is reported at other localities, one 

 of them being Big Coppit Key. As this peat, of course, could not have been 

 formed in the sea at depths exceeding a foot or two, its meaning as to submer- 

 gence is incontrovertible. 



Another line of evidence utilized was the relative induration of the rock. 

 It is established that calcareous sand and mud when exposed to atmospheric 

 agencies and between tides commonly become indurated, whereas from 

 present evidence it seems that such induration does not take place on the 

 sea-bottom. For instance, oolitic muds remain soft and incoherent on the sea- 

 bottom, but when lifted above water-level they become hard. An excellent 

 instance of subaerial induration was seen about 2 miles south of Fort Lauder- 

 dale, on the side of the road to Miami, where there is an elevated beach ridge 

 composed of hard oolite. Not only the general form of the beach is preserved, 

 but its seaward and landward slopes are as easily identified as those of a present- 

 day beach ridge. Vaughan has already based deductions on this kind of 

 evidence in discussing changes of level in the Tortugas, where from the pres- 

 ence of submerged beds of indurated detrital limestone he inferred a lowering 

 of the land surface. 



The occurrence of submerged indurated oolite beneath Biscayne Bay, be- 

 neath the peat beds between Sugarloaf Key and Key West, below the covering 

 of mud on the bank between Key West and Boca Grande Channels, and beneath 

 the mud in Marquesas, has been mentioned. The specimens from Marquesas 

 have the same aspect as the subaerial oolite on Key West, the matrix of some of 

 the grains having undergone secondary crystallization. The deduction from 

 the induration of the submerged oolite is the same as that from the submerged 

 scarps, solution wells, caves, and peat beds, viz, that the last dominant change 

 in position of sea-level was one of submergence. 



There appears to have been a tilting or warping of the reef tract during 

 and perhaps subsequent to this submergence, for the general southwestward 

 slope of the bottom is better explained by tilting than by gradation. Vaughan 

 has several times pointed out that most of the Florida barrier reef stands on 

 the seaward margin of a submerged terrace which extends northward beyond 

 the reef limits. Hawk Channel lies along this terrace between the barrier 

 reef and the main line of keys. Off Miami the maximum depths within the 

 reef line are only about 5 fathoms. In tracing the surface westward to 

 Rebecca Shoal it is found to deepen slightly. Between Key West and Boca 

 Grande Channels depths of 8.5 fathoms are attained; south of the Quicksands 

 the depths are as great as 9, 10, and 11 fathoms, while farther west the maxi- 

 mum depths in the Tortugas Lagoon is 13 fathoms. The tilting or warping of the 

 upper terrace surface southwest of Miami, which has been mentioned, the 

 restriction of the subaerial surface between Key West and Boca Grande Chan- 

 nels, and the disappearance of subaerially exposed oolite west of Boca Grande 

 are all in accord with greater submergence toward the west, as suggested 

 by tracing the floor of Hawk Channel. This conclusion also accords with that 

 reached from the more extensive studies of the geolog}- of the peninsula, 

 which is that there has been a slight general westward tilt of southern Florida. 



Additional deductions of importance may be made from the submarine 

 physiography at depths beyond 10 fathoms, and although the investigations 

 are at present 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 



