52 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
limestone, a vrg pcii rvt f2.5, 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 section 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 niargin 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 gradiilyenlarge until the overlying material, n longer ablhi supr 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 thIe formation of other sinks in close proximity to the first. As old sinks become clogged or partly filled, new sinks form by this process continually enlarging the basin.
Not infrequently a sink forms in or near the bed of a stlreatn When this occurs the lower course of the stream, or a part of1'it. may be reversed. Where many sinks form in succession or through a long period of time the valley of the stream is thereby enak .rgedi and is frequently carried to a level lower than the~ original 'dl Lakes jamonia 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.