14 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
exposed along Tampa Bay represent one phase of this formation. 
The rapids of the Hillsboro Biver are caused by this limestone. The 
formation is found at places along this river for about fifteen miles 
inland. 
HAWTHORNE FORMATION. 
The Hawthorne formation is less distinctly delimited than either 
the Chattahoochee or the Tampa formations. The type locality of 
the formation is at Grove Park about three miles west of Hawthorne 
in Alachua County, where the rock was at one time quarried as a 
building stone. At this locality an effort was also made to grind 
and use certain beds of this formation as a fertilizer, as the rock 
contains at these quarries a small percentage of phosphoric acid. 
The formation includes clays, rocks, and limestones, the latter being 
more or less phosphatic. 
ALUM BLUFF FORMATION. 
The Alum Bluff formation consists chiefly of sands, shell marls, 
clays, and fullers earth. The type locality is at Alum Bluff on the 
Apalachicola Biver. The sands of this formation are often highly 
calcareous, grading into sandy limestones. The marls are often 
highly fossiliferous, and are found chiefly in West Florida. Among 
numerous localities at which marls of this formation are exposed, 
are: Alum Bluff, Bailey’s farm on the Chipola Biver, Oak Grove 
on the east border of Santa Bosa. County, and along the Shoal 
Biver, in Walton County. East of the type exposure the Alum 
Bluff formation is extensively developed. It underlies all of Gads 
den County and is exposed frequently in the numerous streams and 
on the public roads. It is also found in Leon County along the Ock- 
locknee Biver, arid in Manatee County, the fullers earth stratum 
being mined at Ellenton. The formation probably originally cov¬ 
ered a considerable area in central peninsular Florida, remnants 
still being found in Marion and Alachua Counties. 
MIOCENE. 
JACKSONVILLE FORMATION. 
The Miocene in Florida includes two formations, the Jackson¬ 
ville formation and the Choctawhatchee marl. The type locality of 
the Jacksonville formation is at Jacksonville, the first description 
