L28 PROCEEDINGS OF WASHINGTON MEETING. 



The next paper was on — 



111-: ( IIATTAHOncilKK EMBAYMENT. 



BY LAWRENCE C. JOHNSON. 



Looking upon :i map of the Gulf of Mexico one prominent feature, certain to at- 

 tract attention, is a deep bight running up into middle Florida, called Apalachee 

 hay. During Miocene time the coast line was very different. The continent on the 

 Alabama side extended down Chattahoochee and Chipola river- to the vicinity 

 of Chipola, or so as to include part of Jackson county in Florida. The Mariana 

 building stone, which is an orbitoidal limestone of the Vicksburg type, formed the 

 shore during this period. To the eastward at the same period the continent did not 

 reach into the peninsula. The shallow Miocene sea, however, toward the south 

 was close set with Eocene islands in the Suwanee region, their sites now marked 

 by the distinctive deposits of the phosphate belt. 



Erosion in the valley of Suwanee liver and in its western hranches exposes 

 the Eocene orbitoidal limestone in many places, and. strange to say, of a type 

 slightly differing from that of the west, but resembling that of Cooper river, South 

 Carolina. Between these two limestone headlands of the Miocene period lie the 

 greater portion of the counties constituting what is known as middle Florida. 



To avoid conflict with a mountain nomenclature, this ancient extension of the 

 hay of Apalachee may he called the Chattahoochee embayment. The Chattahoochee 

 river doubtless poured into the head of it on the northwest, and constituted then, as 

 it still constitutes, the principal contributor of material for its sediments. 



The general appearance and character of the rocks and fossils of this embayment 

 stamp them with a unity of type. The rocks are all limestones, hut generally so 

 impure as to be often almost sandstone. The older of these beds are more compact 

 and harder than the Vicksburg rocks, and even where not silicified and where not 

 a mere calcareous sandstone the fossils do not retain the original shells imbedded 

 in a softer matrix, but have their lime leached out and their cavities often filled with 

 calcite. These rocks, then, are more insoluble, more unyielding, than other known 

 orbitoidal limestones. Upon this fact depend many of the phenomena of this part 

 of Florida. 



Though spoken of as displaying a unity of type, it not intended to treat the rocks 

 and fossils of the Chattahoochee embayment as identical throughout ; there are 

 variations, which may be exhibited by sections. 



Considering the embayment as having become dry land by the usual process of 

 continental uplift, there is to lie anticipated a general dip toward the south, 

 and observations show as much. Recent studies in Florida have brought out an- 

 other fact, viz, that there is a westerly dip toward the axis of the embayment. This 

 is very obscure in the eastern part, but very manifest in that nearer the Chatta- 

 hoochee river. As a consequence, there is a thinning out of the strata eastward 

 and northward, and a deepening of accumulation toward the west and south. The 

 southward dip is well shown on the river. Descending the river, the last seen of 

 the Vicksburg rocks is about Port Jackson, a short distance above the mouth of 



