20 Transactions. 



tiania type includes iron-ore deposits that appear to have been 

 formed before the solidification of the granitic magma. These 

 ores are never found in the granite, but always in the ad- 

 jacent rocks. If they had been introduced after the cooling 

 of the magma they would also have been deposited in the 

 granite. The eruptive magma is believed to be the source of 

 the metal, which is expelled in the heated steam into the 

 surrounding rocks. 



The synthetic experiments of Daubree seem to justify the 

 views of Vogt, Beck, and other observers that cassiterite and 

 pegmatite veins are formed by gaseous and aqueous emana- 

 tions, and not by direct segregation. 



Art. IV. — Thermal Activity in. its Relation to the Genesis 

 of certain Metalliferous Veins. 



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

 M. (London), F.G.S., Director Otago University School 

 of Mines. 



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



It is manifest that the whole series of eruptive after-actions 

 will commence at the moment of intrusion of the magma, and 

 continue until the igneous mass has become completely 

 cooled. 



Igneous magmas are now admitted by petrologists to con- 

 tain more or less water together with many constituents of a 

 hydrous or gaseous character. Hence the fusion of magmas 

 is not believed to be pyrogenetic — that is, the result of dry 

 heat alone — but hydato-pyrogenetic — that is, fusion by heat 

 in the presence of water. 



According to Arrhenius* water renders the magma more 

 liquid. It has been shown by experiment that magmas which 

 require a temperature of 3,000° Fahr. to produce dry fusion 

 can be fused in the presence of water at 500° Fahr. Accord- 

 ing to the same distinguished physicist water in a rock 

 magma acts the part of an acid, liberating free silicic acid 

 and free bases. 



The activity of water at high temperatures is very great. 

 Barusf has shown that water heated above 185° C. attacks 

 the silicates composing soft glass with remarkable rapidity ; 

 and Lemberg has proved experimentally that water at a 



* Svante Arrhenius, " Zur Physik des Vulkanismus," Geol. Foren. 

 Forh., Stockholm, 1900. 



f C. Barus, " Hot Water and Soft Glass in their Thermodynamic 

 Relations," Am. Jour. Sci. iv, vol. ix, 1900, p. 161. 



