ORIGIN OF THE HARD ROCK PHOSPHATES. 
47 
the hard rock phosphate is a soft phosphate of lime that has the 
consistency of soft, plastic clay. This soft phosphate often under¬ 
lies the hard and is several feet in thickness.” 
Mr. N. H. Darton, writing in the American Journal of Science 
for February, 1891, considers guano as the most probable original 
source of the phosphate. The early Miocene is regarded as the 
probable time of deposition of the guano which by leaching 
supplied the phosphoric acid. Two processes in the formation of 
the rock are recognized. The first is the replacement of the car¬ 
bonate of lime by phosphate of lime; the second is a general 
stalactitic coating on the massive phosphates and in the cavities. 
Whether or not the restricted distribution of the phosphate was 
connected with the genesis of the rock Darton regards as undeter¬ 
mined at that time. 
Mr. Walter B. M. Davidson contributed a brief paper on the 
origin and deposition of the Florida Phosphate, which was 
published in the Engineering and Mining Journal, Vol. 51, pp. 
628-G29, 1891. This paper has not been available to the writer, 
but from a reference in a later paper it appears that Davidson at 
that time believed that the hard rock phosphate boulders were 
deposited in underground caverns and river beds in the Vicksburg 
Limestone. 
Among important early publications on the Florida phosphates 
is a paper by Dr. W. H. Dali, published in 1892. Dali’s account 
of the phosphate was given in connection with and was incidental 
to a general summary of the geology of Florida included in a 
monograph on the Neocene of North America by Dali and Harris 
(Bull. 84, U. S. Geol. Survey). In this report Dali expresses the 
belief that the phosphoric acid of the phosphate deposits was 
derived directly from bird guano. The local character of the bird 
rookeries determine the local occurrence of phosphate rock. The 
influence of local clay beds on the accumulation of workable 
deposits is also recognized (p. 135). 
Davidson, in a paper read before the American Institute of 
Mining Engineers at the Baltimore meeting in February, 1892, 
published in the Transactions, 1893, appears to derive the hard 
rock phosphates as residual material from the Vicksburg Lime- 
