BY LOFTUS HILLS, M.B.E., M..SC. 137 



lias been determined by zonal precipitation, each zone repre- 

 senting certain limiting ranges of temperature and pressure 

 which characterised the conditions during actual deposition 

 of the mineral species from the ore-bearing solutions. The 

 origin of the ore-bearing solutions is ascribed to the differ- 

 entiating igneous mass which gave rise to both them and 

 the underlying granitic mass. 



L. K. Ward elaborated this conception of zonal precipita- 

 tion in a paper read before this Association in 1911, entitled 

 "An Investigation of the Relationship between the Ore- 

 **bodies of the Heemskirk-Comstock-Zeehan Region and the 

 "Associated Igneous Rocks." His conception demonstrates 

 three zones — the Granite Zone; the Contact Metamorphic 

 Zone; and the Transmetamorphic Zone — the latter being 

 subdivided into the Pyritic and Sideritic Belts. The factor 

 determinating the amount and kind of precipitation from the 

 outwardly migrating ore-bearing solutions is the decrease 

 in temperature and pressure as distance is gained from the 

 magmatic hearth. 



It must be here pointed out, however, that A. Mcintosh 

 Reid has recently adduced evidence which shows that the 

 Comstock magnetite deposits, classed as contact-metamorphic 

 by Ward, are magmatic differentiations within the basic 

 phase of the Epi-Silurian plutonic period. There are, how- 

 ever, undoubted contact metamorphic magnetite and hasma- 

 tite deposits around the periphery of the granite, and Ward's 

 conception of zonal distribution is not affected in general 

 principle. 



(3) The Read-Rosebery Zinc-Lead Sulphide Deposits. 



These deposits have been studied in detail by the writer, 

 and their composition, structural features, mineralogy, and 

 genesis are fully delineated in Bulletins 19 and 23 of the 

 Geological Survey. The northern extension of this belt is 

 described by A. Mcintosh Reid in Geological Survey Bulletin 

 No. 28. 



It is shown that the zinc-lead sulphide ore-bodies are 

 metasomatic replacements of schistose calcareous beds in the 

 Read-Rcsebery schist series, which, as previously indicated 

 in this review, are predominantly sedimentary in origin. The 

 component beds of this schist series have been thrown into 

 a series of complex folds by the same stress which brought 

 about their schistosity. The axes of the two series of folds 

 are at right angles to each other, and the more important of 

 these have been mapped. The actual structure observable 



