40 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
at Tiger Bay where it is found to be from 275 to 350 feet thick. Its 
south-eastward extent is indicated by well samples from Fort Myers 
as well as by the secondary deposits of pebble phosphates found 
along Coloosahatchee River and its tributaries. 
Lithologically the Alum Bluff formation is extremely variable. 
At the type locality the formation consists as already stated chiefly 
of calcareous and slightly phosphatic sands. Below the sand is 
found shell marl, while above is found cross-bedded sands and clays 
containing plant fossils. Elsewhere the formation holds clay beds, 
including the workable fuller’s earth beds of Florida. In its east¬ 
ward and southward extent the formation consists chiefly of a sandy 
phosphatic marl, the phosphate being found in the form of pebbles 
and probably also in a finely divided condition. 
The phosphate pebbles in the formation vary in color from shiny # 
black to pure white. In size they vary from exceedingly minute 
pebbles not larger than sand grains to masses two or three inches in 
diameter. In shape they are usually more or less rounded or flat¬ 
tened. The small ones are as a rule nicely rounded, smooth and 
shiny, while the large ones are often angular or very imperfectly 
rounded. The pebbles, especially the larger ones, when broken are 
found to be by no means homogeneous in structure, but include sand 
grains, casts of shells and minute phosphate pebbles. In structure 
the large pebbles are essentially the same as the matrix, and it would 
seem that the pebbles took their present shape and size at the time 
the marl was being accumulated, and that the large ones represent 
mud bails made of the material that was then accumulating in the 
ocean. 
The following analyses are of pebbles from the Alum Bluff for¬ 
mation. Analyses by the State Chemist of Florida. 
Analysis of phosphate pebbles washed from the bed rock of the Pierce Phos¬ 
phate Company, Pierce, Florida. Air dried sample. 
Moisture —---— .97 
Insoluble matter, sand etc. -- 10.75 
Phosphoric acid, 19.41, equivalent to tri-calcium phosphate_ 42.39 
Iron and alumina_ 0.19 
Calcium oxide, 16.79, equivalent to calcium carbonate_ 38.20 
Analysis of phosphate pebble washed from bed rock marl of the Phosphate 
Mining Company, near Mulberry, Florida. Air dried sample. 
Moisture - .94 
Insoluble matter, sand, etc. -.- -6.il 
Phosphoric acid, 24.88, equivalent to tri-calcium phosphate- 54-34 
