232 CARNEGIE INSTITUTION OF WASHINGTON. 



Geologic Investigations of the Florida Coral Reef Tract, by Thomas Wayland 

 Vaughan and Eugene Wesley Shaw. 



The iiiv'estigations of the geology and geologic processes of the Florida 

 reef and key region were continued along lines followed by Vaughan during 

 seven preceding seasons. Additional bottom samples were collected, especially 

 around Tortugas and between Key West and Marquesas. These samples are 

 being studied to ascertain the proportion of calcium carbonate contributed 

 by different agencies, the processes wherebj^ ingredients other than calcium 

 carbonate are added to the bottom material, and the factors controlling the 

 distribution of material according to size. All of these are subjects of prime 

 importance in understanding phenomena associated with coral reefs. 



Shaw resurveyed Loggerhead Key, using the same scale as Vaughan did 

 in 1914. He also made plane-table surveys of Bird, Long, Bush, Sand, 

 Middle, and East Keys, and indicated the position of the present shore-line 

 on the large-scale map of Garden Key. Comparison of these maps with those 

 made by the Coast and Geodetic Survey during previous years shows that the 

 forms and even the positions of the keys are continually being changed, though 

 the rate varies greatly from time to time and from key to key. A sufficient 

 body of data has now been accumulated to serve as a basis for describing the 

 modifications in the key outlines and for referring the modifications to their 

 causes. The current records made in 1914 and the records of the wind move- 

 ment by the U. S. Weather Bureau are essential parts of this information. 



Other problems received attention, but it is not practicable to present the 

 results in a form sufficiently succinct for this Year Book. For example, atten- 

 tion was given to the slopes of the beaches and their controlling factors, the 

 sizing of the sands composing the keys, the reason why the outer border is com- 

 monly higher than the interior portions of the keys, the question of submarine 

 destruction of calcium-carbonate structures by different agencies and the part 

 they play in the development of sea-bottom forms, the transportation of 

 chemically precipitated sediment which is taking place on a large scale, the 

 stage of the Tortugas in the cycle of their history, the composition of water in 

 Biscayne Bay and in the Everglades, the pitting of limestone along the sea- 

 shore, the interpretation of the channel forms of Miami and other rivers, and 

 estimates of the ages of the various physiographic features both in years and 

 in the cycle of their existence. 



Akeal Geology. 



Additions to the knowledge of the areal geology of the region were made 

 between Key West and the western margin of Marquesas. Oohtic limestone 

 was found between Key West and Boca Grande on West Crawfish, Barracouta, 

 Woman, and Man Keys. Except these exposures of indurated oolite, the 

 bank between Key West and Boca Grande channels is composed of calcareous 

 mud, overlying hard rock, which ranges in position from low-tide level to 

 10 feet below that datum. Many soundings through the mud in the Marque- 

 sas lagoon showed the rock surface to lie from 9 to somewhat more than 12 feet, 

 generally about 12 feet, below low-tide level. Vaughan had previously found 

 one locality at which the rock is 15 feet below low tide, with 15 feet of mud 

 over it. The mud ranges in thickness from 4 to 15 feet — for the most part 

 8 to 10 feet. A pipe was driven into the rock below the mud near the west 

 end of Conch Key and a core of indurated oolite was obtained, thus confirming 

 Vaughan's published deduction, from finding hard oolite grains in Marquesas 

 bottom samples, that the foundation of the Marquesas is oolite. How far 

 westward of the Marquesas the oolite extends is not known, but it appears safe 

 to assume that it underlies at least the eastern part of the Quicksands and 

 it may extend to Half Moon Shoal and even to Rebecca Shoal. 



