PEBBLE PHOSPHATE DEPOSITS. 
65 
beds have been subjected to the sorting power of water is of itself 
sufficient to account for such variations as occur. 
VARIATION IN THICKNESS OF THE PEBBLE CONGLOMERATE. 
The thickness of the pebble conglomerate constantly varies from 
place to place. This variation of thickness is due first of all to the 
irregularities of the bed rock. The top surface of the pebble phos¬ 
phate stratum, where it grades into the overlying sands is as would 
naturally be expected much more nearly uniform than is the bottom 
of the stratum which necessarily conforms to the irregularities of the 
bed rock. 
COLOR OF THE PEBBLE ROCK. 
The pebble phosphate in the “bed rock” marl is black, amber, 
brown, gray or white, while the rock itself is prevailingly yellow. 
Accordingly the pebble conglomerate derived from the “bed rock" 
presents a similar range of variation in color. 
VARIATION IN THE GRADE OF ROCK. 
Of the many variations in the phosphate beds none is more per¬ 
plexing nor of more direct concern to the phosphate operators than 
that of variation in the grade of rock. The variation in grade is 
doubtless influenced by many factors some of which as least may be 
accounted for. First of all it must be remembered that deposits that 
have accumulated through the agency of water are necessarily more 
or less assorted, and this is true whether the material is accumulated 
by surface streams or by wave action, or by both agencies combined. 
This partial sorting of materials while the pebble phosphate stratum 
is being accumulated would almost necessarily result in variation 
from place to place in the grade of the rock. 
There is moreover another factor involved in this problem that 
as yet has not been specially mentioned, but which affects the grade 
of rock and also helps to account for some of the other variations 
in the beds. From the generalized geologic section given on page 
53 it will be seen that the Bone Valley formation cuts across the 
eroded bed rock. From this it appears that the pebble phosphates 
are derived not from any particular stratum in the bed rock but 
from many different strata. It is not to be expected that the pebble 
from different strata in the bed rock are of the same grade. It thus 
follows that the grade of rock in any particular phosphate pit will 
