ORIGIN OR THE HARD ROCK PHOSPHATES. 
29 
of clay in the phosphate-bearing formation increases in a general 
way in passing to the south. The exposures in the southern part 
of the area show as a rule more clay than do similar exposures in 
the northern part of the area. The phosphate boulders seem to 
have a tendency to group around and to be associated with local 
clay lenses. Frequently the productive pit gives place laterally to 
barren gray sands. 
Flint boulders occur locally in this formation in some abun¬ 
dance, and occasionally phosphate pits that are otherwise work¬ 
able are abandoned on account of the number of flint boulders 
encountered. The flint boulders are usually oval or somewhat 
flattened in shape and are of varying size, some weighing several 
tons. The exterior is usually of a light color. Some of the 
boulders are hollow and occasionally the cavity is filled with 
water; other boulders are solid, compact and of a bluish color 
throughout. Limestone inclusions are frequent in this formation. 
The pebble conglomerate feature is not of frequent occurrence 
but may occasionally be observed in the northern part of the hard 
rock section. An exposure of flint pebbles may be seen in one 
of the pits of plant number 5 of the Cummer Lumber Company, 
about one mile southwest of Newberry, in Alachua County. The 
matrix at this exposure consists of more or less water-worn frag¬ 
ments of varying size together with round or oval water-worn, 
dark colored flint pebbles. This phase of the formation may be 
seen through a distance of ten or fifteen feet along the side of the 
pit. Water-worn pebbles weighing one or more pounds occur 
occasionally in the northern part of the field. 
The invertebrate fossils are found in the limestone inclusions. 
The vertebrate remains are mixed in with the other materials of 
the matrix. The fossil wood is of rare occurrence, but is 
occasionally found in this formation. 
Phosphate rock, although the constituent of special economic 
interest, nevertheless makes up a relatively small part of the 
formation. The phosphate in these deposits occurs as fragmentary 
rock, boulder rock, plate rock or pebble. The boulders are often 
of large size, in some instances weighing several tons, and not 
infrequently needing to be broken up by blasting before being 
