30 
FLORIDA GEOLOGICAL SURVEY—THIRD ANNUAL REPORT. 
THICKNESS. 
The phosphate-bearing formation is exceedingly variable in 
thickness. In general it is of reduced thickness in the northern 
part of the area. In Suwannee, Columbia, Alachua and northern 
Marion Counties, the formation may reach a thickness of from 30 to 
50 feet, although in places it is much reduced or even absent. 
The maximum thickness of the formation is probably found in 
southern Marion County and in Citrus County. Drillings made by 
the Dunnellon Phosphate Company along the Withlacoochee River 
indicate a thickness of from 60 to 70 feet on the particular tract of 
land being prospected. Similar drillings by the J. Buttgenbach 
Company gave in one instance for the phosphate formation along 
the river, a thickness of about 75 feet. 
Extensive prospecting carried on by the Southern Phosphate 
Development Company near Inverness, indicated for the phos¬ 
phate formation a thickness of 50 to 100 feet; 70 feet being 
a fair average for the particular deposits prospected. It is prob¬ 
able that the depth may in places approach 200 feet, although this 
maximum thickness is probably only local. 
SOURCE OF MATERIALS. 
The very complex and mixed character of the material making 
up the phosphate-bearing formation has already been mentioned. 
The determination of the source or sources of all this material is 
a problem of no little difficulty. A part of the material is of 
chemical origin formed in situ. This applies particularly, in the 
writers’ opinion, to boulder phosphate rock and to flint boulders. 
Of the limestone inclusions some constitute a part of the for¬ 
mation as originally accumulated: others doubtless represent less 
soluble remnants left behind as the surrounding limestone dis¬ 
solved permitting the phosphate stratum to subside and enclose 
them. 
The gray sands find their closest resemblance lithologically 
to the sands of the Alum Bluff formation. Indeed as developed 
locally at many places one scarcely finds characters on which to 
distinguish the gray phosphatic sands of this formation from the 
similar gray phosphatic sands of the Alum Bluff formation, as 
seen at the type locality on the Apalachicola River. That these 
sands are residual from the Alum Bluff formation seems probable 
although the possibility of their origin from some of the later for¬ 
mations must be admitted. That they remain as residual from 
the Vicksburg Limestone the writer cannot believe. 
