PEBBLE PHOSPHATE DEPOSITS. 
59 
rived chiefly from the marl beneath consisted of phosphate pebble, 
pieces of the marl, casts of invertebrates, sand, clay, worn bones and 
teeth. This conglomerate, which is the- land pebble phosphate bed, 
makes up the basal member of the Bone Valley formation. That 
part of the formation above the conglomerate includes the sands of 
the overburden. These as previously noted are as a rule phosphatic, 
although the phosphate is either in a finely divided form or is the 
cementing material in the sand, little or no phosphate pebble or 
other coarse material being included in this .part of the formation. 
The change from the conglomerate to the sands is rather abrupt but 
is not marked by evidence of a time break. 
It is usually found that the coarse material of a conglomerate, 
grades to finer material above. This is true of the pebble phosphate 
conglomerate which as already stated grades into the sands above. 
Within the bed itself, however, it is usually found that the coarsest 
part of the conglomerate is that near the top. The assorting of the 
materials near the base is also frequently less complete than near the 
top of the bed. This seemingly exceptional feature finds its expla¬ 
nation without doubt, in the conditions under which the beds ac¬ 
cumulated. 
Fig. 5.—The accompanying sketch has been prepared to illustrate conditions 
that may have and probably did exist in this region at the time the phosphate 
bed was being formed. The sketch is drawn to represent a stage in the ad¬ 
vance of a sea across a land surface, the country rock of which, a marl, pre¬ 
sents the numerous minor irregularities characteristic of a marl or limestone 
country which for a time has been subjected to sub-areal erosion. The land 
area, not yet invaded by the sea is shown at (1) ; the phosphate bed that is being 
accumulated, shown at (2) ; the bed rock marl, some parts of which still project 
above sea level, shown by (3). That part of the land area not yet reached by 
the sea retains its usual covering of soil and residual material. From that part 
which has been invaded by the sea, however, the soil and loose materials have 
been shifted about, washed and more or less perfectly sorted. The organic 
matter of the soil has largely been removed, while the clays, sands and coarse 
fragments are dropped into the depressions. 
