402 Transactions. — Geology. 



trict. Phosphate-bearing rock is easily overlooked, as witness 

 the deposits in Millburn quarry which lay exposed to the view 

 of all passers for years. The purer phosphorite is often very 

 compact, fine-grained, and hard, possessing also the banded, 

 wavy, and chalcedonic structure characteristic of chert or 

 flinty quartz deposited from thermal waters, for which it was 

 long mistaken at Millburn. 



The calcareous sandstone or limestone overlying and form- 

 ing the closing member of the brown-coal measures is found 

 very widely in both the North and South Islands, as already 

 indicated ; and whenever its surface is weathered and uneven 

 the material filling the irregularities, whether it be hard 

 rock or soft sandy marls, should be submitted to chemical 

 examination for determination of phosphoric acid. 



To become of commercial value a phosphate-deposit should 

 fulfil the following requirements : — 



(1.) Of such magnitude as to justify the erection of tram- 

 ways and other surface plant necessary for development and 

 winning of mineral. 



(2.) Of high grade, averaging not less than 50 per cent, of 

 tricalcic phosphate before dressing. 



(3.) In a position easy of access to a railway or seaboard. 



(4.) Easy to win — that is, in a position in which it can be 

 worked water-free by open cuts and quarrying. The over- 

 burden must also be shallow and easily removed. When 

 it exceeds 20 ft. the cost of stripping runs away with the 

 profit. 



It is only in exceptional cases that it pays to mine phos- 

 phate by underground workings. At Eoss Farm, in Pennsyl- 

 vania, during the year 1899, 2,000 long tons were mined from 

 a stratum 30 ft. thick, 4,000 ft. long, and inclined at an angle 

 of 60° from the horizontal. The stratum was mined to a 

 depth of 300 ft. below water-level, and averaged about 56 per 

 cent, of phosphate. Here the matrix consists of a yellow 

 marl, very easily and cheaply broken. The producers, how- 

 ever, do not anticipate to be able to compete in distant 

 markets with the higher grades of phosphate from South 

 Carolina, Tennessee, and Florida, but look only for a remune- 

 rative local market.* 



* 21st Annual Kept. U.S. Geol. Survey, 1899-1900, p. 494. 



