598 



WILLET G. MILLER AND CYRIL W. KNIGHT 



passing in places into a considerable volume of acidic varieties. 

 The pre-Algoman basic rocks are of greater volume and wider 

 extent than they are usually recognized to be, since they are fre- 

 quently wrongly classed as Keewatin. These basic rocks are 



TABLE III 



Pre-Cambrian Epochs of Ontario and Their Metal Production 



Keweenawan 



Animikean 

 (Algoman) 



Epoch, following basic intrusions, of (a) silver, cobalt, nickel, 

 and arsenic at Cobalt and elsewhere, (b) nickel and copper 

 at Sudbury, and copper elsewhere. Certain gold deposits, 

 not now productive, appear to belong to this epoch. 



Epoch of deposition of "iron formation" as a chemical pre- 

 cipitate. 



Epoch, following granite intrusions, of gold at Porcupine and 

 at many other localities, and of auriferous mispickel. De- 

 posits of galena, zinc blende, fluorite, and other minerals 

 also appear to have been derived from the granites, but 

 some of them were not formed till post pre-Cambrian time. 

 Preceding the intrusion of the Algoman granites, basic 

 intrusives, that appear to be of post-Timiskamian age, 

 gave rise to nickel and titaniferous and non-titaniferous 

 magnetite deposits and chromite. 



Epoch of minor deposition of "iron formation" as a chemical 

 precipitate. 



Granite intrusions probably gave rise to ore deposits which 

 have been removed by excessive erosion as is known to be 

 the case with deposits of later origin. 



Epoch of deposition of extensive "iron formation" as a chemi- 

 cal precipitate among other sediments. 



Keewatin Composed largely of basic volcanic rocks. 



represented by the sudburite of the Sudbury area, by the lampro- 

 phyres of Cobalt and elsewhere, and apparently by the basic rocks 

 of the townships of Dundonald, Reaume, and others where asso- 

 ciated with them are nickehferous pyrrhotite and chromite. 



Owing to erosion, the sequence of metal deposition shown in 

 Table III is doubtless incomplete.^ Iron formation occurs in three 



' Two or three hundred feet more of erosion would have left comparatively little, 

 for example, of the Cobalt silver deposits, of which, it would seem, more has been 

 eroded than has been mined, or even of the great Mesabi iron deposits of Minnesota. 



TiMISKAMIAN 



(Laurentian) 



LOGANIAN 



Grenville 



