ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 149 



upon which they are working are wholly inadequate to provide 

 satisfactory explanations of the evident facts. The result is that 

 ■either all faith in their former beliefs is abandoned, or an appeal 

 is made to the interaction of certain ill-recognised local conditions 

 which have no genetic significance. 



Again, curiously distorted views are sometimes expressed 

 with regard to the location of the proximate source of the metallic 

 ores. Thus, with regard to the origin of tin ore-bodies which are 

 situated outside the boundaries of granite or granite porphyry 

 intrusions, one often hears the contact zone referred to as the 

 ultimate goal of mining operations. It is apparently believed that, 

 •once the boundaries of the igneous rock are reached, the actual 

 source of the mineral wealth of that district will have been dis- 

 covered and that a great concentration of tin ore will be found. 

 This is all the more extraordinary in view of the many occurrences 

 now described in which tin ore-bodies have been traced downwards 

 without interruption from the intruded rocks, through the contact 

 zone, into the igneous rock masses themselves. 



The author proposes in this paper to deal with the question of 

 the relationship of a group of genetically connected ore-bodies to 

 the associated igneous rocks. The ore-bodies in question are those 

 of the Zeehan, Comstock and Heemskirk districts of the western 

 ■coast of Tasmania, and the discussion is restricted to those 

 particular points which bear upon the question of genetic relation- 

 ship, and which may have an application beyond the limits of this 

 particular region. 



II. — Summary of the Geology of the Region. 



The area with which this paper is concerned is structurally 

 complex. The oldest rocks present are believed to be either Upper 

 Cambrian or Lower Ordovician in age, and are referred to as 

 Cambro-Ordovician. They consist of a great series of slates and 

 sandstones, with which are interbedded tuffs and lava flows. At a 

 later period a considerable thickness of silurian sediments 

 — conglomerate, shale, limestone and sandstone — has been laid 

 down upon them. 



Serious dislocations of the crust have followed the period of 

 deposition of the Silurian sediments. The Pre-Devonian forma- 

 tions have been invaded by igneous rocks which are believed to have 

 ascended from greater depths at the beginning of the Devonian 

 period. At this period Tasmania was apparently undergoing 

 ■elevation — the strand-line had certainly suffered a negative dis- 

 placement since the Silurian period. It is not known whether any 

 of the Devonian igneous rocks actually reached the surface. There 

 are no effusive rocks preserved, and a long cycle of erosion appar- 

 ently continued without intermission over all Tasmania until the 

 beginning of the Permo-Carboniferous period. The result of this 

 and succeeding erosion cycles is that the plutonic igneous rocks now 

 outcrop on the west, north, and east of Zeehan, and the dyke rocks 

 penetrate the mining field at a number of places. 



