50 Papers from the Marine Biological Laboratory at Tortugas. 



of the Ocean, is sack-shaped and lies east of Andros Island and west of the 

 bank, on the northern end of which is New Providence Island. Its depth 

 ranges from 795 to 1,200 fathoms. The second indentation, Exuma Sound, 

 is eastward from the Tongue of the Ocean, across the bank and keys ex- 

 tending southward from New Providence Island. This sound is purse- 

 shaped and exceeds i ,000 fathoms in depth. Water i ,000 fathoms in depth 

 is close to the eastern shore of the Bahamas as far north as Elbow Cay 

 on Little Bahama Bank. Eastward of the i ,000-fathom curve the bottom 

 rapidly descends to a depth between 2,000 and 3,000 fathoms. The Bahama 

 Islands are subaerial protuberants above the nearly level, slightly sub- 

 merged surfaces of extensive plateaus which on one or more sides rise pre- 

 cipitously from oceanic depths. 



The water from Gun Key to Northwest Passage, a distance of 67 knots, 

 is only 7 to 12 feet in depth, while the bottom southeast of New Providence 

 Island ranges from awash to 12 or 18 feet below water-level. New Provi- 

 dence Island is mostly a platform from sea-level to 20 feet in elevation, with 

 several east and west ridges standing above it; the highest of these, Nassau 

 Ridge, rises to an altitude of about 100 feet. Andros Island has a similar 

 physiography, a nearly level platform ranging in height from sea-level to 

 20 feet above that datum plane. Along its eastern front, lying near the 

 shore, is a series of interrupted hills that range in elevation from 20 to 60, 

 and perhaps in some instances to 100, feet. West of the coastal hills the 

 surface is low, usually less than 10 feet in elevation, while much of the areas 

 along the west front is affected by the tides. The Barrier Reef of Andros 

 stands on the seaward edge of a shallow, submarine platform, at a distance 

 of one-half mile to 2 miles off shore. The slope of the bottom seaward of 

 the reef is comparatively rapid ; between the reef and the shore the depth 

 ranges from i to 2 fathoms. The platform between the reef and the shore 

 is composed of hard lime rock, the surface of which is indented by numerous 

 submarine pits. The deepest of the holes, locally known as "blue holes," 

 sounded was 36 feet deep. As the general aspect of these holes is similar 

 to the abundant subaerial pot-holes, locally known as "banana holes," 

 formed by solution of the limestone, they are evidence of subsidence. 



The general country rock of the Bahamas is oolite. The sea bottom 

 from Gun Key to Northwest Passage, New Providence Island, and Andros 

 Island is all or mostly composed of oolite, precisely similar to that of 

 southern Florida. The material forming the ridge at Nassau has been wind- 

 blown, and evidently is a dune accumulation. Many or all of the ridges 

 rising higher than 20 or 30 feet above sea-level have been formed of wind- 

 blown material, of which oolite is an important constituent, but the oolite 

 of most or all the lower areas is of marine origin and has not been subjected 

 to aeolian action. 



The finely divided calcium carbonate oozes in the shoal water? of Florida 

 and the Bahamas have long attracted attention. Louis Agassiz, as early 

 as 1851, noted them, as did Captain Hunt, Alexander Agassiz, and others. 



