64 
DEPOSITS OF PHOSPHATE OF LIME. 
[BULL. 44. 
cially true of those nodules which are coated with an enamel, it being 
invariably the case that the hard, enamel like crust is much richer than 
the soft and less compact interior. In this peculiarity the South Caro- 
lina nodules resemble many of those of England and the other phos- 
phate localities in Europe. The phenomenon is dwelt on as one of the 
strongest arguments for the theory originated by Professor Holmes, 
that the nodules have been formed by the phosphatization of lumps of 
Fig. 27. Section ENE. and WSW. through Pinckney's phosphate field, South Carolina. A, sand j 
B, ferruginous sand; C, phosphate rock; D, Ashley marl. Scale: 1 inch = GO feet. 
marl. It ma^, however, be mentioned that a similar condition of the 
nodule could be produced by the action of waters containing carbonic 
acid, which would tend to leach out the carbonate and leave the less 
soluble phosphate of lime. Of course this action would be more pro- 
nounced on the exterior than in the interior of the nodule, and con- 
sequently the result would be a mass much richer in phosphate of lime 
on the outside than in the inside. 
ElG. 
IS. Average section in Pinckney's phosphate mine, Berkeley County, South Carolina. A 
sand ; B, ferruginous sand ; C, phosphate rock ; D, Ashley marl. Scale : 1 inch = 6 feet. 
clay 
The nodule bed varies in thickness from a few inches to about two 
feet and a half, the average being about seven to nine inches. It is only 
when the nodules are found in a pocket or depression in the underlying 
marl that the thickness reaches much overtwo feet, and even under such 
circumstances such a thickness is of very rare occurrence. The yield 
of phosphate per acre varies, not only with the thickness, but also with 
the compactness of the nodular stratum, as sometimes the nodules are 
packed as close as cobblestones in a road, while at others they are scat- 
tered loosely in the sand or clay matrix. The average yield of clean, dry 
phosphate is three hundred to t wel ve hundred tons per acre. The phos- 
phate bed is thought by professor Holmes to be of Post-Pliocene age. In 
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