1922] Whitman: Genesis of the Ores of the Cobalt District 263 



The Keewatin is relatively plastic in contrast with the other forma- 

 tions. This is due chiefly to its varied lithologic character and to its 

 complex structure, which is reflected in the erratic nature of its joints. 

 Certain strong vertical joints, however, occur in expected positions, 

 notably the principal vein joints, and more particularly those near 

 the diabase contact rather than near the sedimentary contact. The 

 faults are particularly characteristic of the formation. All faults 

 passing from other formations into the Keewatin promptly flatten 

 their dips, and frequently change their strikes also. Usually, even 

 though their actual displacements are small, they have considerable 

 gouge, are accompanied by pronounced border zones of breccia and 

 have slickensided walls. Their most significant and important feature 

 is their discontinuity. A given slip diminishes in all directons from 

 a center of maxmum displacement to a periphery of no displacement. 

 Often where one slip ends another begins, lying parallel in an offset 

 position. Also, the fault surfaces are so warped as in no way to 

 approximate planes; and sometimes a fault clean-cut at one point will 

 pass into a set of step faults or a distributive fault at another. 



The Cobalt Series is intermediate in mechanical strength between 

 the Keewatin and the Nipissing diabase. It has the peculiar property 

 of being plastic in one direction and elastic in another. It is well 

 cemented and firm, and is highly elastic to stresses normal to the 

 bedding, certain types of joints having a considerable extension in 

 that direction ; but along the bedding it yields plastically to slight 

 stresses, so that bedding joints and bedding faults are very abundant. 



The diabase is the most homogeneous, elastic, and tough of the 

 formations. Its joints frequently exhibit a conchoidal curvature, their 

 junctions with one another being rounded with mutual branches in 

 the four quadrants. Frequently also a curved or cuplike form is 

 found in parallel joints closely set, like exfoliation fractures, or the 

 layers of an onion. Even the faults often have very sinuous courses, 

 the curves being from five to thirty feet in length. 



All the significant tectonic effects within the district were produced 

 shortly after the injection of the diabase, and presumably ended at 

 a time not long subsequent to the dissipation of its initial heat. Dur- 

 ing this time the chief structures developed were folds, indicating that 

 there were no tendencies toward distension, and there was therefore 

 no opportunity for the formation of gravity or normal faults. As a 

 matter of fact no such faults have been discovered; and all known 

 faults have resulted from compressive stress. 



