124 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



limestone, an average specific gravity of 2.5, a layer one foot thick 

 over one square mile should weigh about two and one-sixth million 

 tons. The calculated rate of removal of this rock is about four 

 hundred tons per square mile per year. From these estimates it 

 would appear that the surface level of the central peninsular sec- 

 tion of Florida is being lowered by solution at the rate of a foot in 

 five or six thousand years. 



With due allowance for a wide margin of error in the above 

 estimates it is still evident that a very great amount of mineral 

 solids is being removed annually in solution. The first effect of 

 solution in limestone is to develop cavities through the rock along 

 the line of ready flow of underground water. These cavities grad- 

 ually enlarge until the overlying material, no longer able to support 

 its own weight, caves in, forming a sink. 



The formation of a sink is a first step in the development of the 

 many basins large and small occupied by these temporary lakes. 

 A sink usually retains connection with the underlying limestone 

 for some time after its formation and water entering the sink 

 escapes into the limestone. Under these circumstances more or 

 less of the material lying immediately around the sink is carried 

 by surface wash through the sink. Moreover the large amount of 

 water entering through the sink results in rapid solution in the 

 limestone of that immediate vicinity. The result is frequently the 

 formation of other sinks in close proximity to the first. As old 

 sinks become clogged or partly filled, new sinks form by this pro- 

 cess continually enlarging the basin. 



Not infrequently a sink forms in or near the bed of a stream. 

 When this occurs the lower course of the stream, or a part of it, 

 may be reversed. Where many sinks form in succession or through 

 a long period of time the valley of the stream is thereby enlarged 

 and is frequently carried to a level lower than the original outlet. 

 Lakes lamonia and Lafayette in Leon County and Alachua Lake in 

 Alachua County are illustrations of basins of this type. 



RELATION OF THE LAKE BASINS TO THE LEVEL OF PERMANENT 



UNDERGROUND WATER. 



It is important to note the relation of these lake basins to the 

 permanent underground water level of the formation into which 

 they drain. It is a well established fact that solution by under- 

 ground water goes on more rapidly above the level of permanent 



