ORIGIN OF CERTAIN CONTACT ROCKS. 187 



In the immediate neighbourhood of the serpentinised mica- 

 gabbro dyke of the Summit Cutting, Comstock tramway, Zeehan, 

 to which reference has been made above, nickel has been detected 

 in pyritic lode matter.^ Nickehferous pyrrhotite is known to occur 

 at two other places in the North Dundas district. ^ 



Niccolite has been found at the Five Mile^ and also in the 

 Zeehan field-* in association \vith silver lead ore. 



It will serve no useful purpose to multiply these examples, all 

 of which appear to the author to support the view that the 

 chromium and nickel contents of certain lodes are ascribable to 

 the chemical action of the ore-depositing solutions upon adjacent 

 basic igneous rock masses. 



Enquiry along the lines here indicated leads inevitably to the 

 consideration of the origin of the iron content, in addition to that of 

 the nickel and chromium, of such ore-bodies as those mentioned. 

 With respect to this question the author cannot here say more than 

 that he believes only a portion of the iron to have been derived 

 from the basic rocks themselves. 



V. — Conclusion. 



The acceptance or rejection of the hypothesis here advanced 

 would seem to depend primarily upon the more detailed observation 

 and explanation of the field occurrences of these rock types in the 

 north-western part of Tasmania along the axes of igneous invasion 

 which have some geological persistence. But much collateral 

 support may be adduced from any other regions in which the sites 

 of basic and acidic invasions have been coincident. The time 

 interval between such invasions of igneous material is a matter of 

 no significance. The acidic invasion must needs be the later, and 

 the liberation of magmatic vapours and solutions from the acidic 

 magma hearths is essential. 



The evidence which may be claimed to have bearing on the 

 whole question is such as may establish in any region the existence 

 of lateral secretion by juvenile or magmatic waters, whether the 

 material secreted give substance to metallic or non-metallic 

 products. 



It is to be expected that such evidence will in most cases have 

 reference to the metallic ores, since the activity of so many geologists 

 is directed towards the investigation of the ores of the valuable 

 metals. The author would, however, venture the opinion that the 

 detailed study of the alteration of basic igneous rock material by 

 acidic_ intrusions of later date will afford much information of 

 scientific value, the economic importance of which will not be 

 negligible. 



1 Vide : G. A. Waller, " Report on the Zeehan Silver-Leaii Miiiins Field," 1904, p. 8:!. 



2 Vide : G. A. Waller, " Report on the Ore Deposits (other than those of tin) of North Dundas, 

 1902. pp. 6i, 66. 



3 Ibid., p. 60. 



iOeol. Sun. Tas., Bulletin No. S, 1910, p. 52. 



