262 



University of California Publications in Geology 



[Vol. 13 



Structures 



Tt has been found possible to follow out roughly folds in rock 

 devoid of stratification and to detect and follow important faults and 

 joints by the evidence found in topography and certain sets of other 

 joints. This is because the joint systems express the structure, and 

 because the chief method of degradation employed by the Pleistocene 

 glacier was the plucking of joint blocks. The walls of faults are 

 strongly jointed, and in consequence of this, the glacial plucking was 

 most active on these lines, sculpturing valleys, and here and there 

 precipitous escarpments, which in many cases overhang. On the limbs 

 of folds, where gently inclined joints were developed by folding, 

 making with oblique vertical joints rhombohedral blocks, the glacial 

 plucking similarly exposed the structures. 



Folds. — All the folding was quite gentle, the usual minimum dip 

 of the slopes being from 10 to 15 degrees, and the usual maximum 

 being from 25 to 35 degrees for both major and minor folds. Prospect 

 Hill on the west side of the town of Cobalt approximates a monocline, 

 the area to the west being essentially flat save at one or two points 

 where a slight westerly dip is visible. Cobalt Hill on the east side 

 of Cobalt Lake is a slightly asymmetric anticline, the eastern slope 

 into Peterson Lake being the gentler. The Lawson Hill immediately 

 east of Kerr Lake, and the McKinley Darragh Hill southwest of Cobalt 

 Hill, are also of a similar character. The Keewatin area between 

 Cart Lake and Contact Bay on Giroux Lake is a nearly flat anticline. 



Of the two sets of tectonic forces which produced these folds the 

 greater acted along a NW-SE axis and tended to produce folds strik- 

 ing NE-SW, as if the especially strong NW-SE structural lines were 

 the outcrops of strike-slip faults between which longitudinal compres- 

 sion occurred. The strength of the lesser set of forces is indicated 

 by minor folds superimposed upon the northeasterly ones, and gen- 

 erally striking northwesterly. As a rule the northeasterly or major 

 folds are the older of the two, but in one or two cases major folds 

 were at least accentuated after the development of the minor folds 

 on their flanks. Although there is thus a simple sequence in the salient 

 events, still in the lesser deformations there were various and indeter- 

 minable alterations of strain from one major axis to the other. 



Faults. — In studying the geo-mechanics of the district it is neces- 

 sary to picture a restoration of the region, and regarding the entire 

 rock mass as a composite medium, to consider the mechanical peculiar- 

 ities of each component formation. 



