A Contribution to the Geologic History of the Floridian Plateau. 133 



The area of Dade County, from which the surface run-off is into the 

 bays and sounds behind the keys, is approximately 1,840 square miles. 

 The surface run-off from this territory would be approximately 0.52 

 cubic mile per annum. This amount, however, ought to be increased 

 as the waters from Lake Okeechobee and the Everglades move south- 

 ward, and a portion of them apparently must flow to the southeast. As 

 is well known, a considerable portion of the territory to the north of 

 Lake Okeechobee is drained into that basin and the water is discharged 

 through the rivers leading to the west, east, southeast, and south of the 

 Lake or the Everglades. Therefore the discharge into the bays and 

 sounds is probably between i.o and 0.5 cubic mile. 



CHEMICAL DENUDATION. 



Having given in the preceding remarks the physical surroundings 

 of the bays and sounds, and given an estimate of the surface run-off of 

 the waters, the chemical denudation of the region may be discussed. 

 That chemical denudation is active in southeastern Florida is attested 

 by numerous phenomena. The surface of the Miami oolite is extremely 

 irregular; some irregularities are due to rocks torn from the general 

 oolitic mass by uprooted pine trees, while others are produced by the 

 solvent effect of water, as is especially well shown by small sink-holes, 

 pot-holes, and such phenomena as the Arch Creek natural bridge. 

 According to Mr. Sanford: 



The holes, which communicate with underground solution channels, are of 

 all sizes, varying from those not over an inch across to those 20 feet or more in 

 diameter. Their depths range from 3 to over 10 feet. Besides the sharply out- 

 lined holes, there are throughout the pineland countless shallow hollows i to 3 feet 

 deep and 10 to 100 feet across. A few of these hollows may owe their origin to 

 original conditions of deposition, some may be due to the overturning of trees and 

 consequent upheaval of the rocks loosened by roots, w^hile others have been caused 

 by the falling in of the roofs of subterranean water-courses. Few of the holes and 

 hollows are large enough to be termed sinks. The large vertically walled holes 

 running down to permanent water-level form natural wells, the shallow hollows 

 are best denominated pot-holes. The writer has heard of only one rock-rimmed 

 opening in southern Florida that resembles the great sinks in the country to the 

 north. 



While there is danger of exaggerating the activity of underground and sur- 

 face water in eating away the soft limestone of the east coast, yet there is plentiful 

 evidence of solution. The pot-holes and the hollow-sounding areas of rock, per- 

 haps 25 feet across, with as many as 6 or 7 holes a foot or so in diameter showing 

 the water beneath, that are fovind along the edges of the southern Everglades, the 

 springs below tide-level at Cocoanut Grove, and other points on the shore of Bis- 

 cayne Bay, the Punch Bowl, a spring basin, the deep holes in New River, and the 

 shallow gorge of Arch Creek with its low rock bridge, all bear witness to the w^ork 

 being done. 



The conditions favorable for vigorous chemical denudation of Hme- 

 stone are: (i) a supply of water charged with CO2; (2) the water re- 

 maining in contact with limestone a sufficient time to permit solution; 

 (3) having dissolved lime to be able to move onward. 



Conditions favorable for such denudation are largely realized in 

 southern Florida: there is limestone; the waters become charged with 



