[TYRRELL] PRE-CAMBRIAN GOLDFIELDS OF CENTRAL CANADA 95 



later ones should be dropped. The fact that a name once correctly 

 given has been wrongly used for years by some or even by all 

 geologists is no justification for rejecting that name altogether, 

 or for refusing to apply it correctly when its proper meaning 

 as originally used has been determined. I therefore agree with Dr. 

 Coleman and Mr. Collins in retaining the name Huronian in the general 

 classification of the Pre-Cambrian rocks of Central Canada. 



The great unconformity between Logan's Huronian and Lauren- 

 Han is clearly shown in the classifications of Collins, Miller, and Cole- 

 man, and is probably indicated by the Eparchaean Interval in that 

 of Lawson, in which latter case the name Huronian is incorrectly 

 applied by him to beds below this Interval or Unconformity . 



From the point ot view of the Economic geologist this unconfor- 

 mity is of the first importance. Above it are great basic intrusives 

 associated with such rich ores of copper, nickel and silver as are found 

 at Calumet, Sudbury and Cobalt, while gold seems tc be conspicuously 

 absent. Below it is an intimately folded and fused mixture of sedi- 

 mentary and igneous rocks, the latter varying in character from 

 basic basalts to highly acid granites. Large areas of these rocks appear 

 to be barren of useful metals of any kind, but where such a metal is 

 present it is not silver, copper or nickel, as in the overlying formations, 

 but it is usually gold. Gold in the Pre-Cambrian rocks of the Cana- 

 dian Shield is thus practically confined to the Pre-Huronian, or to what 

 Sir Wm. Logan evidently intended to call Laurentian, rocks. It may 

 seem to be a heterodox statement to make, but nevertheless it 

 is true, that gold-bearing veins do not occur in the Huronian rocks of 

 Central Canada, when these rocks are correctly designated, but are 

 confined entirely to rocks of Pre-Huronian, or to what Logan would 

 call Laurentian, age. 



The Pre-Cambrian rocks lying below the Huronian in the Cana- 

 dian Shield are somewhat differently designated in the various classi- 

 fication cited above, but that used by Dr. Miller holds most closely to 

 priority of nomenclature, and therefore will be adopted, with the 

 addition of the Couchiching of Lawson. 



In some cases it will be difficult, or perhaps impossible to say with 

 certainty to which subdivision of the Pre-Huronian the gold-bearing 

 veins in any particular district should be allocated. It is true that 

 these veins usually occur in areas characterized by the presence of 

 greenstones or other rocks of Keewatin age, but in many cases it is clear 

 that they have been formed subsequent to the deposition and folding 

 of the later Temiskamian rocks, and in the absence of evidence to the 

 contrary it would appear not improbable that they may have all been 

 formed at about the same late period of Pre-Huronian time. 



