GEOGRAPHY OF CENTRAL FLORIDA 119

 7. THE PENINSULAR LAKE REGION
 (Figs. 18-22, 35, 36, 38. Soil analyses 37-45, C-E, J-M.)
 This is the lar-gest and in some respects the most interesting region in central Florida, with an area of about 4,000 square. miles. It extends along the axis or "backbone" of the peninsula from Clay County to DeSoto. County, and has no counterpart in any other state, though'there is a small lake region in West Florida (described in the 6th Annual Report) that resembles it in some particulars.
 Geology. Geologists have mapped most of the area as underlaid by Upper Oligocene strata, but that is largely hypothetical, for exposure of fossiliferous rock are rare. There are, also patches, belts or pockets of Miocene and Pliocene formations in several places, mostly not far from the St. John's River and its tributaries. Rock Spring, in Orange County (fig. 18) is of interest as being the locality where the first Miocene fossils were found in Florida.* The vegetation in many low places near lakes and rivers seem to indicate limestone or marl near the surface, and there are a few large limestone springs in Volusia, Seminole, Orange and Lake, Counties.
 On the summit of Iron Mountain there is a little. ferruginous sandstone or conglomerate, a kind of rock common on non-calcareous uplands in the coastal plain from New Jersey to Texas, but rare in peninsular Florida. A hard sandy clay, visually pinkish or mottled (but bright red around Lake Wales in Polk County), seems to be nearly everywhere present on the uplands, though natural exposures of it are scarce, for it is usually overlaid by a few to several feet of loose sand. This clay is used in many places for roadsurfacing material, as is some of the marl. Still purer clays are used for brick-making at Whitney, and some kaolin is mined near Okahumpka. There are vast deposits of peat in all the counties (described in some detail in the 3d Annual Report), bordering the larger lakes and rivers and completely filling many of the smaller, lake basins. One or two of the peat bogs in Lake County are rich in diatoms, and have been used in a small way for "infusorial earth."
 Topography. The Ocklawaha and St. John's Rivers are bordered by flatwoods sometimes several miles wide, differing little from

 *See E. A. Smith, Am. Jour. Sci. 121 :309. April, 1881; Tenth Census U. S. 6:190. 1884; Dall & Harris, U. S. Geol. Surv. Bull. 84:125. 1892; Matson & Clapp, 2nd Ann. Rep, Fla. Geol. Surv. 114. 1909.