Park. — The Bock-phosphates of Otago. 401 



The once-famous beds of South Carolina are considered to 

 be of Post-Pliocene age.* 



The phosphate of lime formerly worked at Aruba and 

 Sombrero, in the West Indies, was originally a coral lime- 

 stone converted into a phosphate by the percolation of water 

 containing phosphoric acid derived from the overlying deposits 

 of bird-guano. 



The geological conditions which accompany and doubtless 

 determine the presence of workable deposits of phosphate are 

 the presence of a phosphate-bearing formation at the surface, 

 lying in a favourable position for weathering and subsequent 

 concentration of phosphate by replacement or secondary 

 enrichment. 



To favour the formation of large deposits it is further 

 necessary that the topographical conditions should be such as 

 to favour the weathering of the phosphatic beds over con- 

 siderable areas. Should the phosphate-bearing bed, for 

 example, crop out on a steep slope, the width of exposed 

 surface where the weathering can take place will be neces- 

 sarily limited in extent, the greater part of the formation 

 being protected by the superincumbent strata. Hence phos- 

 phate-deposits left by leaching or produced by concentration 

 on such steep slopes will be narrow, of small extent, and in a 

 position easily removed by denudation. 



On the other hand, where the phosphate-bearing rock is 

 exposed on long gentle slopes, or over an extent of nearly 

 level country, well drained by streams, the conditions will be 

 favourable for the leaching of the rock over correspondingly 

 wide areas, and consequently favour the formation of large 

 deposits. 



So far as known to the author, the discovery of workable 

 deposits of phosphate of lime on the Horseshoe Estate at 

 Clarendon is the first in Australasia, and, apart from its im- 

 portance to the owners, is certain to prove of inestimable 

 value to the agricultural interests of the colony. 



The evidence available from a surface examination shows 

 that a large quantity of phosphate rock exists in this district, 

 but until the deposits have been fully developed by trenching 

 it would obviously be impossible to express the tonnage 

 numerically. 



This discovery will doubtless be followed by other dis- 

 coveries in different parts of the colony in districts where 

 similar geological conditions exist, the most likely localities 

 being in Southland, North and South Otago, North and South 

 Canterbury, Marlborough, Eaglan, and North Auckland dis- 



* Penrose : U.S. Geol. Survey, Bulletin No. 46, 1898, p. 60. 

 26 



