THE FLORIDA PHOSPHATE DEPOSITS.

THE LAND PEBBLE PHOSPHATE-BONE, VALLEY FORMATION.
LITHOLOGIC DESCRIPTION.
The land pebble formation to which Matson and Clapp applied the term "Bone Valley Beds" was briefly described in the Second Annual Report. This formation includes a lower phosphate bearing member and an upper sand or sandstone membe-r. The lower member of the formation contains the workable phosphate deposits. The upper member forms the overburden-which must be removed in mining.
The phosphate bearing member of this formation is more or less definitely stratified, the stratification line being frequently continuous along the full length of the pit, a distance of a half mile or more. Elsewhere the stratification is irregular and cross bedding is evident.
Although variable from place to place this part of the formation has an average thickness of from 8 to 12 feet; its maximum thickness is possibly 18 or 20 feet. The matrix in which the phosphate pebble is imbetlded consists largely of clay, sand and soft/ phosphate. The pebble phosphate makes up in the workable deposits some ten to twenty-five per cent. of the whole. This member shows certain characteristics which are fairly persistent. The lower 2I2 to 3 feet is usually olive green in color, and contains pebble imbedded in clay. The next 3 to 5 feet is frequently dark blue in color although oxidizing on exposure to drab or yellow. The upper 2 to 4 feet of this member differs much particularly in the northern part of the area from that which lies below. This upper part contains coarser material and has a higher percentage of pebble phosphate in proportion to the matrix. The break between the coarser material at the top and the more clayey material beneath is particularly well marked as seen in the pit of the Coronet Phosphate Company in Hillsboro County (P1. 4, Fig 2.) The break is here so abrupt as to constitute a distinct unconformity. The line of contact is marked by the presence of water worn corals, bone fragments and very coarse conglomerate of phosphate pebbles. Passing to the south the contact line becomes less marked, the conglomerate character of the upper part largely disappearing at the south end of the phosphate area.
The indurated sand above the phosphate has an average thickness of from IO to 14 feet. Its maximum thickness, however, is mruch greater. On the other hand owing to decay and erosion these sands are in places much reduced and may be locally entirely

33