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 3o to 5o 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 Rive t indicate a thickness of from 6o to 70 feet on the particular tract of land being prospected. Similar drillings by the J. Buttgenbac. 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 phosphate formation a thickness of 5o to IOO feet; 70 feet being a fair average for the particular deposits prospected. It is probable 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 die writers' opinion, to boulder phosphate rock and to flint boulders.
Of the limestone inclusions some constitute a part of the 
mation as originally accumulated: others doubtless represent less soluble remnants left behind as the surrounding limestone dissolved 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 formations must be admitted. That they remain as residual from the Vicksburg Limestone the writer cannot believe.