84 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
RELATION BETWEEN THE HARD ROCK AND THE 
LAND PEBBLE PHOSPHATES. 
It may be of interest in this connection to note some points of 
similarity as well as of contrast between the hard rock and the land 
pebble phosphates of Florida. First of all both the hard rock and 
the land pebble deposits are contemporaneous in time and are de¬ 
rived from the same source, namely, the phosphatic marls of the 
Alum Bluff formation. However, in the accumulation of the hard 
rock phosphate chemical action has predominated, resulting in the 
formation of boulders of very high grade. The land pebble phos¬ 
phates on the other hand represent primarily a mechanically accumu¬ 
lated pebble conglomerate, although as in the case of the hard rock, 
the grade has been subsequently improved by secondary enrichment. 
The relation of the deposits to the underlying formations is like¬ 
wise different. The hard rock phosphates rest upon the Ocala lime¬ 
stone, the upper Oligocene formations having entirely disintegrated. 
The pebble phosphate deposits on the other hand rest upon the 
eroded top surface of the Alum Bluff (upper Oligocene) formation. 
Land animals are present in some abundance in both formations, 
but it is only in the land pebble phosphate deposits that such dis¬ 
tinctly aquatic forms as cetaceans have as yet been found.* 
*The Zeuglodont taken from the hard rock phosphate deposits referred to 
on an earlier page is residual from the Ocala formation. 
