1922] Whitman: Genesis of the Ores of the Cobalt District 



277 



which the minor one pitches, but also those developed on the limbs of 

 the minor syncline due to its own lesser folding. 



If minor synclines and anticlines pitch down the sides of a major 

 syncline, and folding is renewed on the latter, then the minor anti- 

 clines will act as resistant ribs tending to oppose the folding. They 

 will therefore receive a heavy endwise thrust, and split joints will 

 develop along their axes, the earlier transverse split joints at the same 

 time being closed. However, if this renewed major folding were 

 only slight, it might explain irregular conditions of stress in the walls 

 of the two intersecting sets of joints. This is the anomalous condition 

 under which mineralization took place in the veins of the Crown 

 Reserve and Kerr Lake mines. 



It would seem that these joints must have been produced under 

 conditions of large differential stresses capable of splitting the rock 

 in such extensive fissures against a general three-dimensional stress 

 due to the effect of gravity upon this resilient medium. There was a 

 fracturing force, and a resisting pressure which tended to limit frac- 

 turing to such places and intervals as would give the maximum relief 

 to the differential stresses, with the minimum of openings. It must be 

 inferred from the facts thus far presented that when the vein fissures 

 originated the rock containing them was at a sufficient depth below 

 the surface to be incapable of developing small joints. The measure 

 of that depth can only be roughly approximated. From the nature 

 of fractures generally encountered in mining operations, I would 

 postulate a depth of several thousands of feet ; and this may well 

 be assumed, for the time which elapsed between the invasion of 

 Nipissing diabase and the carving of the surface upon which the 

 local Silurian limestones were laid down, probably includes the Cam- 

 brian and Ordovician periods ; and in that span of time there may 

 have been considerable material eroded from this region. The expla- 

 nation of the existence, position, and character of the gaping fractures 

 must then lie in the reduction of gravitative stress effected by the 

 degradation of the surface. These fractures have opened in the walls 

 of cemented split joints and elsewhere in further relief of the elastic 

 deformation produced during the folding. 



The steep easterly-westerly faults, although in some instances strik- 

 ing due east and west, and frequently departing from that direction 

 by only small angles, vary in important instances from a strike of 

 N 70 W to S 70 W. The major structural axes of the region strike 

 respectively N 35 W and S 42 W, the major folding having occurred 



