160 PROCEEDINGS OF SECTION' C. 



migrating together by way of one fissure from the igneous hearth 

 deposition will take place in the inverse order of the solubility of the 

 metallic compounds. So, under ideally uniform conditions, a 

 mineral-bearing fissure would exhibit a successive series of zones 

 of precipitation in which the different metallic minerals in turn 

 give place to others with increasing distance from the source. 



Different fissures supplied with similar materials will be filled 

 with similar materials ; but on account of the inevitable variation 

 in the conditions controlling precipitation the different minerals 

 will be deposited at varying distances from the source. 



If the author's contention with regard to the superior import- 

 ance of temperature changes in effecting precipitation is correct, 

 and if it be granted that the hypothesis regarding the origin and 

 mode of derivation of the metallic ores is sound in essential par- 

 ticulars, it will follow that in such a case of ore-genesis as that which 

 is being considered : 



(1) Precipitation of metallic ores may take place within the 



boundaries of the igneous rock (the already solid portion 

 of the parent magma) only in those parts which have 

 cooled sufficiently to chill the isuing vapours to the 

 necessary degree. 



(2) Precipitation of the metallic contents of the issuing 



vapours will be selective, and only such minerals will be 

 deposited within the boundaries of the igneous rock as 

 are rendered on the one hand solid by loss of tempera- 

 ture, and are on the other hand insoluble in the vapours 

 or solutions which are being expelled. 



(3) Beyond the boundaries of the heated magma which has 



partially or almost completely solidified there will be 

 successive zones of precipitation, which will have a 

 tendency to follow the isogeotherms at the period of 

 precipitation. 



(4) Where the upper limit of the igneous rock is uneven the 



boundaries of the zones of precipitation will, save where 

 material variations are caused by conditions already 

 cited, be curves somewhat flatter than the isogeotherms 

 of the period — the degree of curvature being reduced by 

 the conditions of pressure, for the isogeotherms will pro- 

 bably conform closely to the igneous boundaries. 



(5) The region in which the several zones of precipitation are 



most likely to overlap will be that in which the successive 

 isogeotherms are most closely crowded together. This 

 region is probably the contact metamorphic zone. 



(6) The region in which the nature of the precipitated mate- 



rial is least liable to variation will be that in which the 

 temperature conditions are most equable. This region 

 is that which is enclosed within the boundaries of the 

 still heated but already solid portion of the igneous 

 magma. 



