164 PROCEEDINGS OF SECTION C. 



The metallic ore-bodies do not lose their character as igneous 

 rocks when they are expelled from the magmatic hearth. They 

 are still as truly igneous rocks as are the apophyses and dykes 

 which are derived from the same magma. The issuing mineral- 

 bearing vapours and solutions, by the alteration of the wall rocks of 

 the fissures which they traverse, are effecting what is really only a 

 special case of contact metamorphism. In cases where there is a 

 metasomatic replacement of the wall rock there is really a special 

 variety of " assimilation," or " magmatic stoping." The 

 mineralising solutions are at a lower temperature than igneous 

 massifs at the time when the contact metamorphic effects are 

 being produced, and are very different with respect to mass. But 

 the author would contend that there is no essential difference 

 between the " alteration of wall rock " and " contact metamor- 

 phism," these terms being employed in their usual senses. 



If such a hypothesis as has here been briefly stated can be 

 accepted in its essential details, and if the several vein -phenomena 

 which are met with in the progress of mining ores of this character 

 are read in the light of such a hypothesis, a more ready a- preciation 

 of the significance of primary variations in an ore-body will be 

 possible. While it is granted that a mine manager is on the whole 

 more concerned with the structural details of the lode which he is 

 working, he should be prepared, by an intimate knowledge of its 

 genetic relationships, to provide sound advice when confronted 

 with the problems introduced by the horizontal and vertical 

 variations in the mineralogical composition of the ore. Such 

 variations may be gradual, or they may be abrupt, but they occur 

 in almost ever type of lode. They are to be understood only 

 through the systematic study of the genesis of the lodes, and a 

 knowledge of the geological conditions existing over a far wider 

 area than that enclosed within the boundaries of a single mining 

 lease. 



EXPLANATION OF PLATE XII. 



The section is essentially a diagram. 



Every endeavour has been made to attain accuracy in all particulars 

 The same scale has been adopted for the vertical as for the horizontal 

 measurements lest exaggeration in any particular arise. 



The points of emergence of the several dykes and veins shown in the 

 section have been projected on to the plane oi the section, which has been 

 chosen in position so that there may be depicted the greatest amount of 

 information pertinent to the objects of the paper in a single diagram. 



The direction of the plane of the section is N. 77 degrees E. (magnetic 

 bearing). 



The Pre-Devonian rocks are shown undifferentiated for the reason that 

 their complex structural arrangement remains still unexplained. But certain 

 faults which antedate the vein-fissures are diagrammatically shown. 



The several subterranean contours are delineated in accordance with 

 the views put forward in this and other papers presented by the author at 

 this meeting of the A.A.A.S. They are entirely hypothetical, being con- 

 structed from the surface evidence and the author's reading of that evidence.. 



