182 Reviews — Pliosphate Deposits of South Australia. 



and vegetable matter, and in collecting this holes and cavities in 

 the underlying limestones were followed down for the guano. At 

 10 feet phosphatic bone-breccias were encountered in the cavities. 



Rock-phosphate. — The deposits of the mainland are found in 

 association with metamorphosed Cambrian or pre-Cambrian lime- 

 stones or in the adjacent argillaceous sediments. The author is in 

 some doubt as to whether the phosphate with which the limestones 

 are associated is a contemporaneous deposit, concentrated by the 

 withdrawal of calcium carbonate in solution from phosphatic 

 limestone, or whether the phosphate has been introduced from some 

 extraneous source at a later date as a metasomatic replacement of 

 the marmorized limestones and calcareous slate. " In their location 

 among the steeply inclined sedimentary beds the bodies are generally 

 close to the limestones . . . but others are found in clay-filled pockets 

 in the strike of the limestone beds. Their position suggests derivation 

 from the eroded portions of the limestones, and that precipitation of 

 the phosphate by replacement took place either on the remaining 

 limestone or in calcareous slate from which the bulk of the carbonate 

 has been removed, the resulting rock being a phosphate with a high 

 insoluble content. Metasomatic replacement is indicated by the 

 presence of older veins of quartz, surrounded by phosphate . . . 

 Another feature indicating metasomatism is the silicification of 

 certain beds at some of the dejiosits to form a chert-like black 

 to brownish-yellow rock containing a little phosphate," which is 

 " frequently brecciated and contorted in places, and is found em- 

 bedded in the phosphate rock. White to greenish sub-translucent, 

 stalactitic, reniform, and mammillary phosphate is common, 

 occurring as coatings of vughs, and wholly or partly surrounding 

 masses of rock-phosphate, and is clearly secondary to the main mass 

 of the rock in its order of deposition. Both limonite and 

 manganese oxide are almost invariably associated with the phosphate 

 rock. It is probable that in the replacement and solution of the 

 rock containing the phosphate deposits, the iron set free has a 

 tendency to segregate at the surface." This theory indicates 

 subsequent replacement, and the author goes on to describe the 

 close association of the deposits as a whole with Tertiary gravels 

 which overlie the deposits. Where the latter have been eroded 

 the deposits of phosphate do not exist. He associates the process 

 of metasomatism with pre-Tertiary drainage, and the destruction 

 of all the previously existing deposits not protected by the Tertiary 

 gravels to post-Tertiary denudation. This impKes that the deposits 

 do not extend to great depth, but they have been proved to depths 

 of 110 feet. He points out the absence of any evidence as to the 

 true age of the phosphatization. 



Apatite. — This occurs at Moonta, Wallaroo, and Balmacoota, 

 in pegmatites of " pre-Cambrian age ". In a vein near Balmacoota 

 Spring the apatite makes up 40 per cent of two veins aggregating 

 5 feet in thickness. 



