108 Papers from the Marine Biological Laboratory at Tortugas. 



The preceding measurements are made along parallels of latitude 

 and are not precisely transverse to the long axis of the Peninsula, which 

 is from N. 20° W. to S. 20° E., but they clearly indicate the approach 

 southward of the loo-fathom curve to the shore, and the persistent width 

 of the submarine portion of the Plateau on the west. In no instance is 

 the width of the subaerial portion of the Plateau so great as that of the 

 subaqueous portion. They are most nearly equal along parallel 27°. 



In the Gulf of Mexico the descent from the loo-fathom curve is 

 abrupt until a depth of 1,500 or 2,000 fathoms is reached. The steepest 

 portion of this declivity is about 60 miles slightly north of west (N. 68^* 

 W.) of the Tortugas, where within 22 miles there is a drop from 100 to 

 1,500 fathoms, and within about 12 miles further an additional descent 

 from 1,500 to 2,000 fathoms. The usual depth to the south in the Florida 

 Straits is over 500 and less than 1,000 fathoms. Off Havana is a tongue 

 of deep water, which the 1,000-fathom curve marks. The Florida Straits 

 are from 500 to 1,500 fathoms shallower than the nearby bottom of the 

 Gulf of Mexico. 



The water between the southern portion of the east coast and the 

 Bahamas is still shallower, being less than 300 fathoms in depth. The 

 500-fathom curve passes around the eastern side of the Bahamas. Off 

 the eastern coast the descent from the loo-fathom curve is not nearly 

 so precipitous or so great in amount as in the Gulf. In the Atlantic the 

 1,000-fathom curve is slightly sinuous, but follows a southerly course 

 from near Hatteras to the eastern side of the Bahamas and forms the 

 eastern boundary of the "Blake Plateau."^ 



THE 10-FATHOM CURVE. 



The lo-fathom curve does not in all places lie close to the shore, 

 indicating extensive areas of shoal water. It is about 25 miles offshore 

 east of Femandina and slightly less opposite Jacksonville, south of which 

 it irregularly approaches the .shore, coming very near it opposite Jupiter 

 Inlet and from there to Key Biscaync. It closely follows the outer edge 

 of the growing coral reefs, extends westward beyond the Marquesas 

 and more than halfway from them to the Tortugas. It also incloses 

 the Tortugas, but does not connect them with the Marquesas. North of 

 the latter keys it bends eastward, lying between 40 and 45 miles west of the 

 mouth of White Water Ba}'. Thence it runs northward to beyond the 

 Thousand Islands, and from there roughly parallels the west coast. It 

 is about 15 miles west of the mouth of Charlotte Harbor, about the same 

 distance west of the mouth of Tampa Bay, 30 miles west of Cedar Keys, 

 10 miles south of Cape San Bias; westward of the last -mentioned locality 

 it is from 5 to 10 miles offshore. (See plate i.) 



If Florida were elevated only 60 feet, all of the area surrounded by 

 the lo-fathom curve would be added to the land surface. The whole of the 

 key region, excepting the Rebecca Channel, would be above water-level, 

 leaving the bottom of Florida Bay dry, as would also be a strip of land 

 25 miles wide opposite Femandina, but narrowing southward on the 

 east coast, and a strip from 10 to 30 miles wide on the west coast east 



' A. Agassiz, Three Cruises of the Blake, p. 96. 



