ORIGIN OF THE HARD ROCK PHOSPHATES. 
51 
residual from the decay of that formation. In answer to this 
hypothesis it may be noted that while the Vicksburg Limestone 
is known by surface exposures throughout a large extent of the 
territory in the Gulf States, and by well borings to a considerable 
depth in Florida and elsewhere, it is strikingly free from inclu¬ 
sions of phosphate rock, such as would remain upon the disinte¬ 
gration of the limestone to form these phosphate deposits. 
Cox, in successive papers, argues that the phosphate rock is 
itself mineralized guano. This, likewise, was the view of Johnson 
(1893), as applied at least to the plate rock phosphates of Marion 
County. The fact that not a few of the phosphate boulders and 
pieces of rock have retained more or less well preserved evidence 
of their derivation from limestone sufficiently controverts this 
hypothesis, which is otherwise improbable. 
Darton (1891) and Dali (1892) each assume that guano is 
the immediate source of the phosphoric acid. Barton’s paper on 
this subject is brief and includes merely a statement of the 
probable origin of the rock. Dali, however, gives a clear state¬ 
ment of the guano hypothesis in its relation to the hard rock 
phosphates of Florida. It is even thought probable by Dali that 
each local deposit of hard rock phosphate may represent the loca¬ 
tion of an ancient bird rookery. The hypothesis of the origin of 
the phosphate from guano fails entirely to account for the 
jumble of materials with which me phosphate is associated. This, 
in the writer’s opinion, is the insurmountable objection to the bird 
guano theory, as developed by Dali. 
Of those who have written on the origin of the hard rock 
phosphate deposits of Florida, no one, with the exception of Eld- 
ridge, has taken sufficient account of the complexity of this forma¬ 
tion, or has seemed to appreciate that it is as necessary to account 
for the associated materials as for the phosphate itself. With 
the hypotheses proposed by Eldridge, however, the writer is un¬ 
able to agree. 
Whatever the original source of the phosphoric acid, whether 
from guano or from phosphate of lime, originally disseminated 
throughout the Vicksburg Limestone, the subsequent process, 
according to Eldridge, was the formation of a highly phosphatized 
