156 ■ PROCEEDINGS OF SECTION C. 



The amount of the metallic ingredients present does not appear 

 to the author to offer a means of distinction, since the metallic 

 minerals present (chiefly cassiterite and pyrite) are at times very 

 sparsely distributed. No quantitative limit can be drawn between 

 these veins and dykes composed of the same minerals in ever 

 varying proportions. 



It does not seem possible to determine definitely how much 

 of the vein-matter (or dyke-matter) is restricted by the boundaries 

 of the central fissure, and how much has been distributed by metaso- 

 matic replacement in the adjoining country. Different occur- 

 rences vary in this respect. 



The author would therefore submit that there is no essential 

 difference between a dyke and a vein of this character. Different 

 dykes or veins are characterised by the presence of different 

 amounts of mineralising agents and different amounts of metallic 

 ingredients. Such variations as are recognisable appear to the 

 author in no way to invalidate this conclusion. 



If this interpretation of the field relationships between the 

 igneous rocks and the metallic ores of this region is correct, we are 

 in a position to establish a number of most important deductions : — 



(1) The actual source of the metallic ores is located within 



the heart of the earlier consolidation products. 



(2) The period of expulsion from the igneous hearth coin- 



cides with the final stages of consolidation. 



(3) The metallic veins and igneous dykes of the latest 



stages of consolidation are essentially identical in 

 mode of origin, in place of origin, in composition and 

 in structure. They are indistinguishable, and if we 

 refer to one series as dykes, the other should receive 

 the same name. 



(4) In the case of th':s series of genetically related ores, all 



the members, whether remote from the source or in 

 immediate proximity to it, must be regarded as direct 

 products of the consolidation of the igneous magma — 

 i.e., as true igneous rocks, or, in view of their usual 

 tabular form, as dykes. The silver-lead lodes of 

 Zeehan are to be regarded as dykes in precisely the 

 same way as are the quartz-tourmaline-cassiterite 

 lodes of Mt. Heemskirk. 



(5) The metallic ore veins, the dykes, the quartz-tourmaline 



nodules, and the normal granite of the Heemskirk 

 massif are consolidation products of different stages 

 in the differentiation of a single magma. The existing 

 end-products of the process of differentiation are on the 

 one hand the granite, and on the other hand the sideritic 

 silver-lead veins. These end-products we may readily 

 distinguish as igneous rocks and as mineral veins 

 respectively ; but no such distinction can be drawn 

 between the latest-formed dykes and the roots of 

 the vein series. 



