PEBBLE PHOSPHATE DEPOSITS. 
69 
SECONDARY ENRICHMENT IN THE PHOSPHATE BED. 
The manner of accumulation, structural detail and local varia¬ 
tions of the land pebble phosphates having been described, there yet 
remains for consideration the very important factor of secondary 
enrichment within the bed itself. That secondary enrichment has 
occurred in the phosphate bed is shown by the fact that the phos¬ 
phate pebbles washed from the bed rock contain only from 42 to 
58 per cent tri-calcium phosphate, while those washed from the 
phosphate bed contain from 66 to 77 per cent tri-calcium phosphate. 
In fact this tendency to phosphatization is general in the beds, and 
with the exception of oyster shells and pieces of wood, which are 
silicified, has affected the pebbles and rock fragments, the bones 
and teeth of vertebrates, and the casts of the invertebrates. 
The agency through which secondary enrichment has been ac¬ 
complished in this formation is with little doubt the surface waters 
which move downward through the phosphatic sands of the over¬ 
burden, and laterally through the phosphate bed. A similar process 
contributed as has been shown in a previous paper to the enrichment 
of the hard rock phosphate. 
Secondary enrichment of phosphate rock is doubtless due to the 
fact long since demonstrated by Reese* that when waters holding 
calciunl phosphate in solution, come in contact with calcium car¬ 
bonate, a reaction takes place by which the phosphate replaces the 
carbonate. In view of this fact it seems not at all impossible that 
the phosphate pebbles of the bed rock itself may have their origin on 
this process of enrichment, the original pebbles of sandy marl having 
under favorable conditions become phosphatized possibly from phos¬ 
phoric acid in solution in the sea water. 
*Chas. L. Reese, Amer. Journ. Sci., 3rd Ser., Vol. 43, p. 402, 1892. 
