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University of California Publications in Geology [Vol. 13 



parallel to the latter axis. If before deformation, a circle had been 

 drawn about the area at the horizon of the present surface, after 

 deformation it would have had the form of an ellipse whose longer 

 axis would have been only slightly greater than the short one, and 

 would have had a strike of perhaps N-S. The two diameters connect- 

 ing the four points of intersection of the circle and the ellipse might 

 then have had strikes approximating N 70 W and S 70 W respectively, 

 these being axes of maximum shear. This may explain the mysterious 

 easterly-westerly faults. The difficulty with this explanation, how- 

 ever, is that it implies contraction in one horizontal direction and 

 elongation in another, whereas the escape of material due to the two- 

 fold compression must have been upward. It would seem more satis- 

 factory to regard these faults as expressions of the resultant of the 

 two horizontal compressive stresses as they acted upon non-homo- 

 geneous media, the variations in strike representing the intensity 

 fluctuations of the forces. At any rate these faults seem to be closely 

 tied up with the deformative forces which accompanied the cooling 

 of the diabase, since they traverse it, and in two instances, where they 

 meet the Cobalt Lake fault, are dislocated by it. One of these is on 

 the La Rose property northeast of Cobalt Lake, and the other, on one 

 of the Hudson Bay claims southwest of the Lake. The fact that such 

 faults sever the diabase sheet, as in the O'Brien and Beaver mines 

 (although scarcely dislocating it), need cause no confusion in this 

 regard, since the compressive stresses caused by the radiation of mag- 

 mat ic heat were generated not only in the diabase mass itself, but also 

 in the heated rocks adjacent. 



Summary. — The significant relationships of time and structure in 

 the ores of the Cobalt District probably center about the advent of 

 the diabase. The Cobalt Lake fault in its earliest activities dates back 

 to the early part of the erosional epoch preceding the laying down of 

 the Cobalt Series, the supposed Timiskaming fault dates back at least 

 to pre-Silurian times, and each of these faults represents one of two 

 oblique complementary systems of lines of deep-seated diastrophic 

 adjustment. These axes of strain presumably determined the warped 

 surface along which the diabase sheet opened its way during injection, 

 as it assumed its present position, lifting the roof rock a distance of 

 1000 feet. Its advent was accompanied by deformations due to the 

 heat-expansion of the neighboring rocks which were slowly raised to the 

 same temperature for a considerable distance back from the magma. 



6 C. K. Leith, Structural geology, p. 16. 



