36 Transactions. 



calcite or even orthoclase, and that rutile and anatase are com- 

 mon as secondary products after ilmenite, titanite, titaniferous 

 magnetite, biotite, &c. Substitution of this kind is pseudo- 

 morphic rather than metasomatic. Upon these and other 

 mineralogical replacements which he enumerates he implies 

 that the formation of vein-filling was the result of replacement 

 molecule by molecule. 



Lindgren thinks this genetic theory may be fully sufficient 

 for many veins, but admits that for many others, perhaps the 

 majority of fissure-veins, there seems to be something lacking. 



Vogt* classifies the metasomatic alterations caused by the 

 circulation of ore-solutions as follows : — 



(a.) Alterations forming greisen, cassiterite rock, &c. 



(b.) Scapolitisation. 



(c.) Propylitisation. 



(d.) Kaolinisation. 



(e.) Sericitisation. 



(/.) Carbonatisation (with dolomitisation). 



(g.) Silicification. 



(h.) Zeolitisation. 



(i.) Intense contact metamorphism. 



The tin-bearing rocks at Mount Bischoff, in Tasmania, are 

 eurite and felsite wholly or partly replaced by massive topaz. 

 To the list of metasomatic alteration of rock-masses must 

 therefore be added topazisation. 



Vogt agrees with Emmons, Becker, Lindgren, and others 

 that metasomatic replacement plays an important part in the 

 formation of mineral veins and ore- bodies. 



Art. VI. — The Deposition of Mineral Matter from Aqueous 

 Solutions in its Relation to the Filling of Cavities and 

 Vein-fissures. 



By Professor James Park, M.A.Inst.M.E., M.Inst.M. and 

 M. (Lond.), F.G.S., Director Otago "University School of 

 Mines. 



[Read before the Otago Institute, 13th September, 1904.] 



The deposition of metalliferous and mineral matter from 

 underground solutions may be effected by one or more of the 

 following causes : — 



(1.) A decrease of temperature. 



(2.) A decrease of pressure. 



* Prof. Vogt, "Problems in the Geology of Ore-deposits," loc. cit , 

 p. 660. 



