166 PROCEEDINGS OF SECTION C. 



constantly in mind when general problems concerning ore depo- 

 sition are under discussion ; for in those cases in which ore deposits 

 have been formed in a single region at different periods a certain 

 confusion may result^. 



It is regarded by petrologists as an established fact that periods 



. of igneous activity have been, throughout geological history, 



coincident with periods of crustal deformation, and that the sites 



of crustal movement have been the sites of co-ordinate igneous 



invasion^. 



By a number of mining geologists a parallel claim is made, viz., 

 that periods of ore deposition have in like manner corresponded in 

 time with the periods of igneous invasion^. 



The coincidence of mineralised areas with areas marked by 

 igneous invasions has already been referred to. Hence relationship 

 in time must be considered with relationship in space. 



The igneous rocks which are found associated with ore deposits 

 are, in different places, products of different phases of igneous 

 activity. In the present paper the author proposes to briefly 

 discuss the form and relationships of certain products of the plutonic 

 phase from the points of view indicated above. 



The question is one requiring discussion in the light of data 

 to be provided by many different metalliferous provinces. The 

 present paper has been written in the light of the evidence presented 

 by certain granite masses which play an important part in the 

 geological history of Northern and Western Tasmania. Inci- 

 dentally the intrusive rocks which are to be referred beyond doubt 

 to the same source as the plutonic rocks are mentioned, and all are 

 showTi together upon one of the sketch maps which accompany 

 this paper. 



II. — The Proximate Source of Igneous Rocks and Metallic 



Ores. 



Concerning the ultimate origin of igneous materials little can 

 be said that is not based upon theories of cosmogony ; *and since 

 the primary source of all metallic ores must needs be igneous 

 material,^ the ultimate origin of the ores cannot be traced back 

 with certitude through the more or less hypothetical stages of the 

 early history of the earth. 



We must be content for the present to accept only certain 

 broad generalisations with regard to the composition of the earth 

 as a whole. With respect to the distribution of heat and the 

 physical conditions in the interior we are not, for the purposes of 

 this paper, concerned. 



1 A. M. Finlayson {loc. cit. supra.) seeks to avoid such confusion by introducing such a term 

 as a " metallogenetic inlier." 



2 A. Harker : " The Natural History of Igneous Rocks," p. 13. 



3 This correspondence in time is in most cases really the most conclusive proof of the gene*'** 

 relationship between the igneous bodies and the associated mineral deposits. 



4 Vitie A. Harker: " The Natural Historv of Igneous Rocks," pp. 3 and 4. 



5 Vide J, F. Kemp: "Genesis of Ore Deposits," A.I.M.E.. p. 684, ar-^ manv other writers 

 Passim. 



