28 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
irregular. In general clay lenses in the phosphate matrix are most frequent in the upper part of the formation.
The red clayey sand called "hardpan" by the miners may be present or lacking in the pits of this section. The loose pale yellow sand is practically always present varying in thickness from I to 25 feet.
MARION COUNTY.
The plate rock deposit found in the vicinity of Anthony and Sparr in the north central part of Marion County represents an eastward extension of the phosphate-bearing formation. The relation of the phosphate matrix to the underlying limestone is the same as previously described. The limestone projects into the phosphate matrix as rounded peaks. (P1. 5.) Circular depressions similar in appearance to pot holes or to "natural wells" are frequent in this section. These through subsidence 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 below 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 below water level are often removed by solution; above this level they are 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 removed by solution. The stratification lines in the plate rock are frequently much curved and distorted owing to this irregular subsidence. (P1. 2, Fig. 3.)
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 small amount of boulder rock. In other words the mechanically transported rock in this section predominates over the rock formed chemically i situ. 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