.4 Contribution to the Geologic History of the Floridian Plateau. 159 



and 420 feet below the surface. The arenaceous composition of the 

 Miocene near the southern edge of the Plateau has already been stated, 

 but may be repeated. 



LITHOLOGY. 



Lithologically the Miocene of the western area (the Choctawhat- 

 chee formation) and that of the eastern (the Jacksonville formation) 

 are decidedly different. The former is predominantly arenaceous, the 

 sands are of a greenish color, weathering yellow or reddish, with an 

 abundance of well-preserved fossil shells, overlain at Alum Bluff by a bed 

 of plastic clay ; the latter contains light-colored, impure, arenaceous lime- 

 stone beds, particularly near the top, with a large amount of argillaceous 

 material, varying in color from light gray to pale yellow. 1 There are 

 sands and clays below the limestone beds of the Jacksonville formation. 

 The thickness of the Choctawhatchee formation varies from 25 to 50 

 feet, while that of the Jacksonville may be from 400 to 500 feet. 



MIOCENE CORALS. 



Attention has not been called to the changes in the coral fauna of 

 the Miocene from that of the preceding Apalachicola Group. The change 

 in it is more striking, if not more important, than that in the mollusks. 

 Reef corals abounded in the older beds of the Apalachicola Group. 

 Corals of that type become rarer in the younger deposits of that group, 

 and are entirely absent in the Miocene formations. The change is dra- 

 matic in its intensity. 



SHORE-LINE. 



In reconstructing the marine conditions of Miocene time the approxi- 

 mate shore-line must be determined, and the focal point of interest is 

 the Suwanee Strait. As has been stated, no Miocene deposits are defi- 

 nitely known between Tallahassee and Trail Ridge. The altitude of the 

 exposures near Tallahassee is about 100 feet ; near Trail Ridge they attain 

 a height, according to the map, of 200 feet. Intervening altitudes are 

 above 150 feet, as at Lake City and Houston. Dall says: 



West of Jacksonville, at Live Oak, Suwanee County, and Lake City, Colum- 

 bia County, specimens of fossils were obtained which may prove to belong rather 

 to this (Jacksonville formation) than to the Chattahoochee group of beds. (U. S. 

 Geol. Surv., Bull. 84, p. 125, 1892.) 



This is a surmise and not an opinion, and later work has not verified 

 the surmise, but it seems probable that Miocene deposits may have 

 extended across this intermediate area. Matson and Clapp say: 



At the close of the Oligocene the State of Florida appears to have had the 

 same general form that it now has, though its area was doubtless less than it is at 

 the present time. With the inauguration of the Miocene there came a submergence 

 which appears to have reduced the land area to a narrow strip along the northern 

 end of the State, and a peninsula which was shorter and narrower than it is at pres- 

 ent. During part of this period the central portion of the peninsula may have 

 been separated from the mainland by a shallow strait. The exact extent of the 

 encroachment of the sea during Miocene times is difficult to determine because 



1 Matson and Clapp, op. cit., p. 108. 



