,4 Contribution to the Geologic History of the Floridian Plateau. 113 



northward to Dunnellon at a distance of from 5 to 12 miles from the 

 coast; thence it bends to the northwest and west, keeping from 10 to 25 

 miles from the shore in the interstream areas, while it digitates up the 

 various rivers and other streams. 



If Florida were depressed 50 feet the greater portion of the Penin- 

 sula would be submerged — or most of Florida is less than 10 fathoms 

 above sea-level. 



The 100-foot contour is not a continuous line around the whole 

 Peninsula, but circumscribes disconnected areas. One of these areas 

 lies west of Kissimmee River, extends from Summit in Marion County 

 southward to near Zolfo Springs in De Soto County, and has its western 

 boundary determined by Hillsboro and Withlacoochee rivers. 



There is another ridge, rising above 100 feet, between Dunnellon 

 and Dade City on the west coast. Between Dunnellon and Hawthorne 

 are other but isolated areas above the 100-foot level. From Gainesville 

 northward, lying between the valleys of St. John's and Suwanee rivers, 

 is another area above this level. West of the Suwanee River the 100- 

 foot contour is present on all interstream lobes. 



If Florida were depressed 100 feet there would remain of the present 

 land surface of the Peninsula four large and a number of smaller islands. 



The 150-foot level is attained in each of the four areas surrounded 

 by the 100-foot contour. Haines City is situated on a ridge extending 

 southward from near the northern boundary to near the southern boun- 

 dary of Polk County. West of Dade City is another ridge over 150 feet 

 in elevation. Between Dunnellon and Hawthorne are isolated peaks 

 above this level. Between St. John's and Suwanee rivers on the north 

 considerable areas are circumscribed by this contour. 



The areas circumscribed by the 200-foot contour are much smaller. 

 Haines City, De Soto County, is located on one of them, and south of 

 that town the map indicates five others on the north and south ridge 

 through Polk County. Four hills over 200 feet in altitude are in northern 

 Pasco County, west of the longitude of Dade City. The only other 200- 

 foot elevation on the Peninsula is the Trail Ridge, along the north and 

 south boundaries of Baker-Duval and Bradford-Clay counties. 



The only 2So-foot elevation indicated on the Peninsula by the map 

 is south of Haines City in central Polk County. 



If the Floridian Plateau were depressed 250 feet (about 42 fathoms) 

 only one small island, about 5 miles long and 2.5 miles wide, situated 

 in central Polk County, would remain of the present land surface of the 

 Peninsula. If the loo-fathom curve is taken as the submarine limit of 

 the Floridian Plateau, it immediately becomes evident that in vertical 

 measure the submarine exceeds the subaerial portion by 58 fathoms: 

 or xff is below water, while yjy is above water. 



In the preceding account, the remarks have been confined to the 

 relief of the Peninsula of Florida, as the westward extension belongs 

 more properly to the continental mass forming the northern boundary 

 of the Gulf of Mexico. In this portion of the State there are higher ele- 

 vations than on the Peninsula; at Mount Pleasant is a small area over 

 300 feet in elevation, and considerable areas are above 250 feet. Mossy- 



8 



