ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 161 



(b). The Application of the General Principles to the Area 

 under Consideration. — When we turn to the particular area with 

 which this paper deals, a number of difficult problems arise, any 

 solution of which must of necessity involve certain hypotheses with 

 regard to the subterranean structure and composition. In the 

 solution which is offered by the author no small weight has been 

 given to the evidence afforded by the neighbouring regions in which 

 it has been his fortune to make geological investigations. 



The first question is concerned with the exact source of the 

 metallic ores of Zeehan, Comstock, and Heemskirk ; and the answer 

 will largely depend upon individual interpretation. 



The author holds the behef that the several ores of this area 

 were not derived from a single point of origin, but that they were 

 derived from several centres of segregation within the heart of the 

 acidic magma. For the area containing the lodes is a large one, and 

 the lodes of the eastern portion descend, in most cases nearly verti- 

 cally, towards different subterranean regions. Yet it does not 

 appear probable that, with the exception of the centre, whence the 

 stannite-bearing lodes were derived, the metallic segregations in 

 the different centres were materially the same. The ore bodies 

 derivable, upon the hypothesis of the author, from similar segrega- 

 tions far outweigh the exceptions both in bulk and in importance. 



Exact equality in the proportions of the different metals 

 (characteristic of different zones of precipitation^) at the different 

 centres appears to be immaterial when the resultant ores are re- 

 viewed as a group ; since separation into approximately constant 

 types will result through the operation of the chemical and physical 

 laws to which reference has been made above. Thus, if much tin 

 is present the cassiterite lodes from such centres will be richer, while 

 the great part of the lead is carried beyond the zone of precipitation 

 of the tin. The types in general will thus be constant. 



But when metals such as lead and silver, the compounds of 

 which are co- zonal as regards precipitation, are concerned, the 

 resultant ore-bodies will have their contents determined by the 

 proportions of these metals present in the centre of segregation. 

 Thus the silver content of the galena lodes of Zeehan derived from 

 different centres will be always variable for this reason alone.^ 



Of the immediate causes of precipitation, the fall in temperature 

 would seem to have always been the chief. The other controUing 

 conditions which have been mentioned do not call for special 

 mention. In the principal productive portion of the Zeehan field 

 the path for the passage of the mineralising solutions has been 

 determined at a number of places by the intersection of lode fissures 

 with certain fault zones or ntscheln, and at these places precipitation 

 has been most marked. This precipitation is the result of the 

 restriction of the solutions to the more open portions of the fissure 

 rather than the direct effect of changes of temperature and pressure, 



1 Tin and lead are thus characteristic of distinct zones in the great majority of cases. But it is 

 recognised that in exceptional cases the ores of these metals do actuall)- overlap. 



2 Silver being a precious and useful metal, its distribution comes more prominently into notice 

 in mining. The exact percentages of the baser metals are but seldom available with such minuteness, 

 and the same weight cannot be attached to them in the comparison of lodes. 



