[tyrrell] PRE-CAMBRIAN GOLDFIELDS OF CENTRAL CANADA 109 



During the peiiod of vein formation the rocks were subjected to 

 very considerable strain, and openings, or lines of flowage for solutions, 

 were formed in them both vertically and horizontally. In these 

 openings large veins or masses of quartz were gradually deposited, 

 either in the openings themselves or by replacement in the adjoining 

 wall rock. 



In other places in the Porcupine District, but especially to the 

 south of the outcrops above mentioned, quartz-porphyry, varying in 

 some places to a grey granite, is rather prominently exposed. It is 

 generally massive or nearly so, and in it, usually near the contact of 

 older greenstone, are often narrow veins of quartz which may occa- 

 sionally carry gold. These veins differ from those of the HoUinger 

 and Dome zones, for while the fissures in which these latter were de- 

 posited were caused by the dislocation and separating of the various 

 layers of the schist through the influence of pressuie, the fissures in 

 the massive porphyry do not show any evidence of having been formed 

 by pressure, but they were probably formed by the shrinking of the 

 porphyry on cooling at the time when the gold-bearing solutions were 

 exuding from the parent magma. The source of the gold was doubtles? 

 the same, but the mode of formation of the two classes of fissuies in 

 which the quartz and gold was deposited was different. 



Swastika. Fifty miles south-east of Porcupine is the district of 

 Swastika and Kirkland Lake, where, for a number of years, gold- 

 bearing veins have been known to occur. The rocks which underlie 

 this country, arranged in increasing age from above downwards, are 

 as follows: 



Diabase 



Lamprophyre & Minette 1 ., 



Albite Diorite / ^^^^"^^^ 



Graywacke 

 Conglomerate 



Temiskamian 



Iron Formation or Jaspylite ] 



Diorite-porphyry j' Keewatin & Laurentian ? 



Greenstone 



Keewatin. 



Greenstones. — Beginning with the oldest, the basement rock of 

 the district is a fine-grained greenstone of Keewatin age, which 

 in many places is hard and massive, while in other places it is slaty or 



