148 Papers from the Marine Biological Laboratory at Tortugas. 



GENERAL RELATIONS OF GEOLOGIC BOUNDARIES IN FLORIDA. 



The preceding account of the areal distribution of the geologic 

 formation according to successive ages has been given to show how the 

 rocks of Vicksburgian age form an eccentric nucleus, on whose northern, 

 eastern, and southern slopes younger geologic formations have been laid 

 down. The next younger group extended northward into Georgia, in 

 which State the Vicksburg again comes to the surface; but toward the 

 east, southeast, and south in Florida it is overlain by later sediments. 

 The boundaries between older and younger sediments are roughly con- 

 centric to the Vicksburg nucleus on the east and south, tending to widen 

 their areas to the south. This statement may appear doubtful, but when 

 it is recalled that Apalachicola sediments extend from the northern end 

 of Tampa Bay to below Sarasota Bay, a glance at the map will show 

 the greater width along a north-and-south line in this area than along an 

 east-and-west line through Palatka. A curved line would have to be 

 drawn from Daytona to Lake Flirt on the Caloosahatchee to show the 

 widening of the Marine Pliocene toward the south. South of St. Augus- 

 tine the Miocene has been buried by Pliocene and later sediments. 



The present coast line preserves this relation to the Vicksburg 

 nucleus, but with the southward extension there has been a flattening 

 opposite the eastern convexity of the old nucleus. 



DRAINAGE LINES. 



An examination of the map of Florida with reference to the drainage 

 lines immediately shows that the State may be divided into two areas. 

 In the extension westward from Aucilla River, a region actually a part 

 of the main continental mass lying north of the Gulf of Mexico, the 

 stream courses are normal to the Gulf Coast. The other area is the Pen- 

 insula portion of the State. In the latter there is a general conformity of 

 the stream courses in the vicinity of the Vicksburg nucleus to its outline, 

 while away from it they more or less parallel the east and west coasts. 



The streams of the Peninsula are those of special importance for 

 this discussion. Santa Fe River runs westAvard near the northern 

 boundary of the Vicksburg, to its confluence with the Suwanee River, 

 whence the latter stream trends southward across the northwestern 

 corner of the Vicksburg area. Between the headwaters of the Santa Fe 

 River and the Ocklawaha is a comparatively low divide with an elevation 

 of very little over ioo feet. Ocklawaha River follows near the eastern 

 boundary of the Vicksburg-Apalachicola from Lake Griffin northward 

 to the latitude of Nashua; then it bends abruptly eastward and flows 

 into St. John's River. Between the headwaters of the Ocklawaha and 

 those of the Withlacoochee and Hillsboro rivers is a region of low relief, 

 in which, excepting a few hills, no place attains an elevation of ioo 

 feet above the sea. Hillsboro River approximately parallels the south- 

 eastern boundary of the Vicksburg-Apalcahicola groups. These data 

 show that there is from the confluence of the Santa Fe with the Suwanee 



