THE HEEMSKIRK MASSIF. 175 



with the igneous dykes and ore-bodies arranged along a zone 

 extending from Heemskirk through Middlesex (perhaps even to 

 Beaconsfield). Along this zone the granite itself outcrops at 

 several points, the granitic apophyses at intermediate points ; 

 while the ore-bodies are distributed along a practically continuous 

 belt of mineral fields. At some depth below the surface there is 

 possibly complete continuity of the granite along this zone. 



(4) The relationship to certain of the structural features of 

 Tasmania. — While the actual form assumed by the Heemskirk 

 massif, its apophyses, and its associated dykes and veins are all to 

 be ascribed very largely to local structural conditions, the general 

 distribution of these deriv-atives of the ascending magma has been 

 decided by the existence of an old zone of crustal weakness. For 

 no less remarkable than the lineal definition of this zone is its 

 geological persistence. The granite of Heemskirk lies upon an 

 axis of at least two igneous invasions, viz., those of Cambro- 

 Ordovician and Devonian time. Still another invasion (of gabbro, 

 norite, and serpentine) affected some portions of this zone at a 

 period which is of Post-Silurian but pre-granitic age. 



With the acceptance of this statement of the structure and 

 relationships of the granite of the Heemskirk Range, many pro- 

 blems of interest and of economic importance suggest themselves. 

 The author cannot here undertake the discussion of such problems, 

 but desires to lay stress upon the necessity for regarding the granite 

 massifs of western Tasmania as being themselves of limited extent 

 in a vertical direction, and extending horizontally along lines of 

 crustal weakness which are axes of igneous invasion. 



The author desires to acknowledge the receipt of no small 

 amount of information with regard to the distribution of the rocks 

 here mentioned from Mr. W. H. Twelvetrees, from whom he has 

 obtained permission to publish the accompanying maps. 



EXPLANATION OF PLATES. 

 Plate XIIL 



The sketch map of a portion of the western coast of Tasmania has been 

 prepared to show the granite massifs, the acidic dykes and the distribution 

 of the mining fields. 



The circles indicate the positions only of the obser\ed outcrops of the 

 dykes. The direction of elongation of these is in many cases unknown. 

 Hence it has been impossible to represent them by conventional symbols. 

 Their magnitude at the surface is very variable and cannot be represented 

 on the map. Thus the outcrop at Mt. Bischotf exceeds in mass the total 

 of all the others shown. 



The position of the principal mining fields is shown by names only, these 

 being located centrall}- in each case. 



Plate XIV. 



All the known granitic massifs of north-western Tasmania are here 

 shown in their relative positions. The relative sizes of the several outcrops 

 are shown in all cases but those which occur at Anderson's Creek. In this 

 place the several outcrops are all small, and they have been collectively 

 shown by a single mass to comply with the demands of the scale of the map. 



