The Phosphates of America. 49 



from the mouth of Broad River, near Port Royal, in the south- 

 east, to the head waters of the Wando River in the northeast. 

 Its major axis is parallel to the coast, and its greatest width is in 

 the neighborhood of Charleston. 



Whether the deposit is continuous or not over the whole of this 

 zone,, it certainly varies considerably in depth and thickness. In 

 many places we have seen it 3 feet thick and cropping out at the 

 surface, whereas in others it has dwindled down to a few inches, 

 or was found at depths varying from 3 to 20 feet. These two con- 

 ditions, thickness of deposits and depth of strata, taken together 

 with the richness of material in phosphoric acid, are of course the 

 chief points for consideration in the economic working of the 

 Charleston phosphate beds on an industrial scale. 



The most approved and generally adopted method of ascertain- 

 ing the importance and value of the deposits is that of boring and 

 pit-sinking. 



A careful topographical survey is first made of the country, 

 and when this has been done there commences a systematic series 

 of bore-holes from any point that may be arranged, by means of a 

 long steel borer or rod, specially designed for the purpose. The 

 boring rod is worked down through the upper strata until it is ar- 

 rested by the solid bed of phosphate. Directly the slightest resist- 

 ance is offered to its passage it is drawn up, and the distance it has 

 traversed is measured with a foot-rule. The measurement having 

 been noted, the rod is again let down, is forced through the resist- 

 ing strata, and is then again withdrawn and measured. The differ- 

 ence between the first and second measurements is taken as repre- 

 senting the thickness of the phosphate bed. These bore-holes are 

 practised at distances of 100 feet apart over the total surface to be 

 examined. The results obtained with the rod are verified and con- 

 firmed by a series of exploratory pits 10 feet long by 5 feet wide 

 which are dug over the course of the bore-holes at intervals of 

 500 feet. The bore-holes are driven to a maximum depth of 15 

 feet, and no pits are at present sunk on those portions of the land 

 where at that distance no phosphate has been encountered. Im- 

 mediately after removing the overlying strata the phosphate is 

 carefully taken out, its depth and thickness measured, and an aver- 

 age sample of the rock and nodules secured and laid aside for 

 analysis. 



The practically invariable nature of the superincumbent ma- 

 terial throughout the entire belt, as shown by the digging of a. 



