GEOGRAPHY OF CENTRAL FLORIDA 95

4. THE PENINSULAR LIME-SIN'K OR HARD-ROCK PHOSPHATE REGION
 (Figs. 8-11, 40. Soil analyses 6-9.)
 This extends from a few miles north of the northern boundary 01 the state southward through the western half of the peninsula to the neighborhood of Tanipa. Its southern limits are ill-defined, or ai least insufficiently explored, but there is at least one area of considerable size in Hillsborough County, entirely disconnected from the rest. It reaches the coast in Pinellas County, which seems to be the only place in peninsular Florida where any high land otnei than dunes and shell mounds can be seen from the ocean. Its area in central Florida is about 2,400'square miles.
 Geology. The greater part of the area is underlaid at no greai depth by a comparatively pure limestone now regarded as of uppei Eocene age, which is practically the oldest rock outcropping In Florida. Toward the southern end of the region this is supposed to dip southward and be overlaid by the Tampa limestone, of Oligocene age. Extending nearly the-whole length of the region are irregular deposits or pockets of hard-rock phosphate, apparently derived mostly from a re-working of the underlying rock by geological processes, but containing many vertebrate fossils of Pliocene age, and designated by geologists as the Alachua formation. Practically the whole surface is covered by several feet of incoherent sana whose age is problematical, and there may be a stratum of clay between the sand and rock in some places, not as extensive ill central Florida as farther north, however.
 The underground water, tapped by many artesian wells at deptnls usually from 50 to uoo feet below the surface, is good to drink. but unsuited for boiler purposes on account of the large amount of limestone dissolved in it. For this reason the Atlantic Coast Line R. R. uses water-softeners at its tanks at Ocala Junction, Dunnellov and Croom, and rain water cisterns are used in some of the towns
 Topography and Drainage. The highest elevations known are a little over 200 feet above sea-level. The topography is. everywhere undulating, with many basins of various sizes and shapes, presumably formed by the solution of underlying limestone. Some of these have sinks or caves in their bottoms, some are sandy and always dry, some are inundated part of the time, and some contain permanent water, making ponds or lakes (fig. io). The dry basins