162 PROCEEDINGS OF SECTION C. 



The feature of greatest prominence which is revealed by the 

 geological examination of the whole area is that concerned with the 

 zonal distribution of the different types of lode matter about the 

 exposed granite. Yet, from what has already been stated, the 

 lodes of the Zeehan field do not proceed from the centre of the 

 Heemskirk massif, but from some hidden source below the field itself. 



It remains, then, to investigate the exact nature of the zonal 

 distribution of ores, and the significance of the granite at a distance 

 from the position of the source of the lodes situated (in Zeehan) 

 on the fringe of the area discussed. 



From the presence of the granite porphyry and gabbro dykes 

 in the very heart of the Zeehan fields it must be granted that there 

 exists some continuity of the acidic and basic rocks at some un- 

 known depth below. No mine workings have penetrated to them, 

 nor has the zone of contact metamorphism which lies above the 

 granite been exposed in depth, for both acidic and basic rocks out- 

 crop to the eastward at North Dundas, and there can be little 

 doubt about the continuity of these in depth through the inter- 

 mediate area. The igneous magma has apparently risen to a greater 

 altitude in the western portion of the area (Heemskirk) than it has 

 in the eastern (North Dundas),^ but in no place have we evidence 

 that sohdification took place actually at the surface. Though 

 buried beneath a surface cover at the time of irruption, the highest 

 peaks of the irregular surface of the igneous rocks have been laid 

 bare at the surface through the subsequent removal of the invaded 

 rocks. In the troughs or depressions of the surface of the igneous 

 rocks the older rocks still remain, since the total effect of the erosion 

 cycles has not yet sufficed to expose continuous areas of the igneous 

 ma.sses. 



The basic magma of the whole region has had practically no 

 contact metamorphic effect upon the intruded rocks, but the acidic 

 magma which succeeded it is surrounded by an aureole of metamor- 

 phosed rocks at the border of the exposed granite. It must be con- 

 sidered certain that the aureole visible at the present surface is 

 merely the exposed outcrop of a zone which once completely 

 covered the granite surface. It has been partially lost by denuda- 

 tion where the actual granite appears, and it dips down below the 

 unaffected members of the invaded rocks, and is lost to view as 

 it follows the granite surface underground. 



Thus the geological plan gives precisely similar information 

 to that which would be afforded by a vertical section. A complete 

 section could be drawn with approximate accuracy if we were aware 

 of the profile of the granite below the contact aureole. The 

 successive visible zones of precipitation which lie horizontally 

 beyond the contact zone are really but the intersections of inclined 

 zones (which conform closely to the granite boundaries) with the 

 surface determined by epigene agencies. 



So the nature of the variations in the lode matter, as observed 

 in a horizontal direction, is sensibly identical with that which would 



1 Compare the extraordinary differences of altitude attained by the basic rocks in the North 

 Dundas district, to which reference has been made by the author Geol. Surv. Tas., Bulletin No. 6, p. 24 



