48 Bulletin 15 358 



the Orbitoides limestone, were found to extend not only over the 

 greater part of northwestern Florida, but also to form, in part, the 

 countr}'' rock of the central portion of the peninsula. The extent 

 of the Chattahoochee is indicated on the map. 



The Orbitoides and Chattahoochee limestones vary in their 

 surface charaAeristics, but the difference is only one of degree. 

 The Chattahoochee is much less porous, denser, more argillaceous 

 and, hence, less soluble than the Orbitoides. As a result, the 

 drainage in the Chattahoochee areas is more on the surface instead 

 of underground, as in the older limestone areas. The more 

 variable beds of the Chattahoochee rock also gives rise to a more 

 rugged topography, which forms a contrast with the level Vicks- 

 burg areas. 



The ChattaJioochee series in central Florida. The Hawthorne 

 beds. In 1887, Dr. Dall described* certain beds of broken-up 

 phosphatic rock, enclosed in a younger matrix, found overlying 

 the Vicksburg group in the vicinity of Gainesville. Beds of 

 the phosphatic rock were later found in place on hilltops near 

 Archer. Dr. Dall has named them the Hawthorne beds, since 

 they are quarried at that town. They usually comprise a 

 greenish clay at the top, followed by ferruginous gravel and 

 phosphatic oolite. The Hawthorne beds constitute the base of 

 the Chattahoochee group in central Florida. 



The Orthaulax bed. Dr. Dall propo.ses the name Orthaulax 

 bedf for the siliceous outcrop on the shores of Tampa Bay, which 

 has been generally called the Tampa silex bed. The original 

 limestone has been much modified by the infiltration and decompo- 

 sition of silex and, locally, by disintegration into marls. The de- 

 posit extends up the Hillsboro and Manatee Rivers. It was ob- 

 served at the Falls of the Hillsboro River, in 1846, by Lieut. John 

 H. Allen, who described! it as a dark, siliceous rock, noted for 

 the beauty of its chalcedonized fossils. He distinguished clearly 



* Loc. cit., p. loS; see also Ainer. Jour. Set., 3rd ser., 1887, vol. 34, p. 164. 

 ■\ Ibid., p. 107. 



J Geology of Tampa Bay, Amer. Jour. Set., 2nd ser. , vol. i, 1846, pp. 

 38-42. 



