ORIGIN OF THE HARD ROCK PHOSPHATES. 
35 
are frequent in this section. These are filled with the phosphate matrix. 
One of these depressions observed by the writer had been cut into, in the 
process of mining. This depression was about three and one-half feet 
in diameter at the top, fifteen feet deep and narrowed gradually to the 
bottom. Other depressions variable in diameter and in depth occur. The 
limestone lying near the line of the underground water level has usually 
a rough and jagged surface owing to solution by water in contact with 
the limestone. Above the water level the limestone has a smooth rounded 
surface, the shells and other fossils having been eroded off plane with the 
general rock surface. The plate rock beds show evidence of having been 
originally faintly stratified. Much of the stratification that originally 
existed, however, has been destroyed through repeated local subsidence 
as the underlying limestone was moved by solution. The stratification 
lines in the plate rock are frequently much curved and distorted owing to 
this irregular subsidence. 
The chief difference noted between the plate rock and the typical hard 
rock region is in the relatively large amount of fragmentary phosphate 
rock and the small amount of boulder rock. Flint and limestone boulders 
chemically formed are likewise absent or rare. 
The deposits at Standard and at Juliette, in the western part of 
Marion County, are similar in general character to the hard rock deposits 
as previously described. The mines in this section are dry mines and 
usually reach to the bottom of the phosphate formation in places en¬ 
countering the limestone. 
In the southwestern part of Marion County and in Citrus County the 
hard rock phosphate-bearing formation reaches its maximum thickness. 
The underlying limestone is ordinarily encountered at a considerable 
depth from the surface. Many of the phosphate pits in this section are 
worked as dry mines to the underground water level and afterwards as 
dredge mines to such depth as the dipper will reach. Some of the pits 
on higher lands are mined as dry mines only. 
The pit at the Dunnellon Phosphate Company plant No. 10 was one 
of the first pits regularly worked in the phosphate section and has been 
continuously in operation for the past twenty years. This mine is operated 
by a dredge. The bottom of the phosphate is not reached in this pit and 
the full thickness of the formation at this place has not been reported. 
citrus COUNTY. 
The conditions in Citrus County are in a general way similar to the 
conditions in the vicinity of Dunnellon, in Marion County. The under¬ 
lying limestone is occasionally seen in the pits in this section md is 
frequently reached by the dredge. The surface of the limestone wherever 
