ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 163 



be visible in a vertical direction could we but follow a single lode 

 downwards towards its source, if the materials supplied to the 

 several fissures throughout the area have been approximately the 

 same. 



The general result appears to have been that cassiterite ores 

 have been precipitated within the igneous boundaries by the 

 ascending vapours and solutions which, as they traversed tht 

 successively cooler zones, have deposited in turn magnetite or 

 mixed magnetite and sulphides, then pyritic zinc and lead ores, and 

 finally sideritic lead ore. Inequalities in the surface of the granite 

 mass have caused the precipitation of these different ores at different 

 levels, and subsequent denudation has exposed the deposits of 

 different zones of precipitation at the level of the present surface 

 in zones of distribution which conform to the boundaries of the 

 granite.^ 



VII. — Conclusion. 



The main object of this paper has been to draw the attention 

 of mining engineers and geologists towards the fact that the inter- 

 relationship between an ore-body and its parent magma needs 

 more careful study, and that an ore-body of such a kind as those 

 which have been mentioned is in reality only a variety of igneous 

 rock. 



For the author believes that greater prominence should be 

 given to the fact that the original magma whence the ore was 

 derived contained both the non-metallic and metallic products of 

 consolidation. Thus the lead ore of Zeehan is not derived from the 

 granite ; but both lead ore and granite are co-derivatives of a 

 magma which contained them and other minerals as well at the 

 time when consolidation first began. 



■ In the case of the magma which has been under consideration, 

 the different products of consolidation result from the operation 

 of two main processes which are materially distinct : — 



(1) The process of the separation of the constituents of 



the metallic compounds from the main mass of the 

 non-metallic portion within the boundaries of the 

 magma. The process is one of progressive expulsion 

 of these metallic ingredients from the solidifying non- 

 metallic aggregate. 



(2) The process of the solidification of the different metallic 



compounds thus separated, when they are released from 

 the bondage of the intra-magmatic foci into which 

 they have been gathered, and are enabled to ascend into 

 regions of lower pressure. Under the varying physico- 

 chemical conditions to which the issuing compounds are 

 subject during their ascent, selective consolidation will take 

 place, partly within the boundaries of the magma and 

 partly beyond. 



1 The hypothetical arrangement of these zones of precipitation is shown in the diagrammati* 

 section which accompanies this paper. 



