GEOGRAPHY OF CENTRAL FLORIDA 163

rocky type in the hammock belts. Those with permanent water in them are apparently more common in northern than in central Florida, but examples can be seen near Sumterville and Lacoochee. In the Hernando hammock belt some of the intermittent lakes or prairies have a small rocky lime-sink at one end or edge, through which the water drains off (fig. 16). There are said to be some lime-sinks on the west side of Lake George, which the writer has mapped as being in the lake region, but not yet explored.
 Caves. Limestone caves are not uncommon in and near the hammock belt in Marion County, and there are a few small ones in the southeastern part of Citrus County,* hardly large enough to contain stalactites and stalagmites or to be easily entered.
 Natural bridges in central Florida are of two types, which might be called wet and dry. The former is the commonest, and is caused by a stream entering a subterranean channel made by solution of lime stone, which it may follow anywhere from a few rods to a few miles. It is of course impossible to go under such a bridge, and sometimes one cannot even be sure where a disappearing stream emerges again. Bridges of this type are reported near Homosassa and Tarpon Springs, and there must be many unrecognized ones made by small streams. A rarer and very different type is formed by blocks of limestone falling against each other when the ground under them settles irregularly from the slow solution of still deeper calcareous strata. A few of this kind can be seen in the neighborhood of the caves of southeastern Citrus County just mentioned.
 Flatuoods. Most of the country within twenty miles or so of the coast on both sides of the peninsula is essentially level, except where shallowly dissected by streams. The dissection is most pronounced near the Peace and Alafia Rivers, and at certain points near the coast where the gene ral level of the country is 20 or 25 feet above the sea, as at Eau Gallie, Melbourne, and St. Petersburg. The flatness is probably due to the fact that the sand and underlying materials were deposited on a nearly level ocean bottom, and have not been elevated high enough or long enough to be eroded much.
 Bcaclics and dunes. The whole Atlantic coast of central Florida and the Gulf coast in Pinellas County, are bordered by rather
 *For additional information about these see R. M. Harper, Am. Fern Journal 6:68-81. 1916; Natural History (formerly American Museum Jou.aal)
2 :201. 1919; J. K. Small, Jour. N. Y. Bot. Gard. 21:34-37. 1920.