66 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
be influenced by the grade of the pebble in the bed rock stratum from 
which it chiefly received its supply of pebble, the grade being fur¬ 
ther modified as already stated by washing and sorting in connec¬ 
tion with the redeposition of the pebble. 
Still another factor that is even more difficult to estimate is that 
of secondary enrichment of the rock after it has reached its present 
location. That the secondary enrichment of the pebble has been 
more effective at some localities than at others is doubtless true, 
even though the particular conditions that have brought about this 
result may not be understood. ■This subject is discussed in a suc¬ 
ceeding paragraph. 
SINK HOLES. 
Sink holes, although not numerous in the land pebble phosphate 
beds, are occasionally met with in mining, one such having been 
seen by the writer in the pit of the Pierce Phosphate Company at 
Pierce. The break in this sink is through both the overburden and 
the phosphate bed, as is shown not only by the fact that the phos¬ 
phate bed drops abruptly, but by the presence of “slickensides” 
formed as the clay moved downward into the sink. 
• 
STREAM BEDS. 
In the same pit and near the sink, an old stream bed was cut 
through in mining. In the bed of the stream is found fine, loose, 
more or less well stratified sand, which is usually dark, although in 
places it is gray or light colored. This stream had cut down to the 
phosphate matrix, and at one point had almost cut out the coarse 
or upper part of the deposit; in other words the stream had cut 
almost entirely through the Bone Valley formation. Subsequently 
owing to changed conditions the stream valley had been filled with 
sand. It is quite possible that both the sink hole and the stream bed 
were formed during the time of the Pleistocene elevation when the 
land stood higher than at present. 
CLAY BEDS BENEATH THE PEBBLE PHOSPHATE CONGLOMERATE. 
In pit No. io of the Prairie Pebble Phosphate Company near 
Mulberry, some 12 or 15 feet of clay matrix carrying but little phos¬ 
phate was found to lie beneath the pebble conglomerate, the sump 
hole of the pit being sunk into the clay. This clay matrix apparently 
does not represent the bed rock but belongs with the Bone Valley 
