168 PROCEEDINGS OF SECTION' C. 



It is highly probable that this statement, advanced with confi- 

 dence for the region mentioned, is applicable also to the other 

 occurrences shown on the maps, in which like igneous rocks are 

 associated with like ore bodies. With regard to the particular 

 region mentioned the field evidence appears to be particularly- 

 conclusive. 



III. — ^The Structure of the Heemskirk Massif. 



(a) The visible contours. — From the progress of erosion in the 

 region to which special reference is here made, we are enabled to 

 form a reasonably complete mental picture of the upper portion of 

 the sum total of the consolidation products of the magma as they 

 would have appeared before erosion — -with all the intruded rock 

 masses stripped from them — -in the district immediately adjacent 

 to the Heemskirk Range. 



The granite massif of Heemskirk appears to have possessed 

 an arched or dome-like surface, from which certain tongues or 

 narrow ridges (of granite porphyry) extended upwards. 



Traversing the smooth surface of the granite, and even cutting 

 across the apophyses of granite porphyry, the tabular dykes and 

 veins of later date have formed ribs or flanges standing out far in 

 relief above the granite boundaries. 



This much we may deduce from the study of the present 

 features of the granite massif, its borders, and the immediately 

 adjacent country. 



{b) The invisible contours. — (i.) In a horizontal direction. — 

 When the immediate vicinity of the exposed massif is left, the 

 mental construction of the form is to be made only by the applica- 

 tion of inferences to be drawn from observation. The upper 

 boundary of the granite dips below the intruded rocks and has not 

 been encountered in mining operations. But the subterranean 

 extension of the granitic portion of the magma beyond the limits 

 of the exposed massif of Heemskirk may be, from field evidence, 

 regarded as certain. 



Dykes of granite porphyry are found at several points in the 

 Zeehan district, and must of necessity imply an underground 

 extension of the magma in that direction. No less significant, in 

 the opinion of the author, is the distribution of the mineral veins, 

 which are, according to his view, also derivatives from the same 

 mother magma. 



The horizontal contours of the invisible extensions of the 

 granite massif may thus be to some degree determined by the 

 observation of the distribution of the dykes and veins at the 

 surface, if the hypothesis of the author with regard to the genetic 

 relationship between the ore-bodies and the igneous rocks is true. 

 The limitations of such a method of mapping the underground 

 extensions are obvi'ous. The amount of erosion subsequent to 

 consolidation, the depth of the granite below the present surface, 

 the vertical extent of the dykes and veins, and the ease or difficulty 

 with which surface phenomena may be examined, all influence the 

 degree of success which any such attempt may attain. 



