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



265 



strikes. On these the known shifts vary from one foot to approxi- 

 mately 100 feet, making various angles with their dips on the surface 

 of movement. Some of them are on bedding planes in the Cobalt 

 Series, and can scarcely be detected at certain points except by the 

 measured shift on them. Others are more obvious, carrying as much 

 as four inches of selvage, and having crushed and jointed walls ; but 

 the amount of crushing seems to be quite unrelated to the amount 

 of displacement, apparently being more closely connected with the 

 kind of rock. Often, also, striae on the fault surface plainly indicate 

 the direction of slip. 



Attention was called in a previous paragraph to the remarkable 

 evenness of the original undeformed Keewatin surface upon which the 

 Cobalt Series was laid down. It must be understood that this is to 

 be taken in a broad relative sense. It is not conceivable, for instance, 

 that it could approximate the evenness of a surface of sedimentation, 

 but must necessarily have had small and perhaps gently undulating 

 relief, the details of which could not now be defined, except by the 

 fractures they would cause in the overlying formation as it was shoved 

 over them during folding. These erratic fractures would tend to 

 confuse the evidence gathered for use in other connections, without 

 being of any value in themselves. Also, local strains would be set up 

 by this means, which would be expressed as joints, sometimes, doubt- 

 less, of such a nature as to constitute vein fissures. These erratic and 

 misleading fractures have caused a certain amount of error in ore 

 predictions ; but their influence in comparison with the other factors 

 is usually small. 



A distinct set of easterly-westerly faults is to be found in all parts 

 of the district. They usually dip at angles varying from 45 to 90 

 degrees, the steeper ones predominating. They usually carry from 

 one-half inch to six inches of gouge, and frequently have strongly 

 striated walls, the striae often being perfectly horizontal. Now and 

 then one set of striae is found crossing an older set. In one case a 

 vein of silver and smaltite had been deposited on a strongly striated 

 fault, and two subsequent movements had striated the ore, so that a 

 hand specimen exhibited two sets of striae making an angle of ten 

 degrees with one another. They are not. strike-slips nor oblique slips 

 in the strictest sense, but range from one to the other. The shifts 

 usually vary from a few inches to perhaps 50 feet; and they cut 

 through all formations and folds. 



