8i 



this diabase in numerous localities throughout this region, 

 the diabase and the ores are believed to have come from the 

 same magma. 



Nearly all varieties of the rocks forming the sill at 

 Cobalt, when examined in thin sections, are found to have 

 an ophitic texture, and primary quartz is almost always 

 present. The rock is, therefore, a quartz-diabase. Most 

 of the quartz is associated with feldspar in micrographic 

 intergrowth. 



The chemical composition of certain typical specimens of 

 the quartz-diabase of the Cobalt area, and its relation to the 

 quartz-norite of Sudbury are shown in a following table. 



The thickness of the diabase sill at Cobalt is five or six 

 hundred feet or more. In diamond drilling, at one point 

 near the shore of Cross lake, the thickness was found to be 

 nearly twice as great, but this is believed to be due to fault- 

 ing. Cross lake lies in line with Kirk, Chown and Goodwin 

 lakes, the cbain of lakes probably indicating the direction 

 followed by a fault. 



The accompanying generalized section shows the re- 

 lation of the diabase sill to the Keewatin and the Cobalt 

 series, and to the veins, in the Cobalt area. Cross-sections 

 of the area published by the Ontario Bureau of Mines give 

 more details, as the following notes on the general section 

 of the area show. The " Map of Cobalt Area," scale 800 

 feet to 1 inch, that accompanies this guide book, shows the 

 location of the sections. 



General Section, Upper Hale of Plate IV. 



The section incorporates much of the information con- 

 tained in other sections, together with additional data. Its 

 total length is about 4^2 miles, and it may be added that 

 the bottom line represents sea level. The cross-section 

 begins at the southeast corner of Sasaginaga lake, and 

 shows the important area of conglomerate, greywacke, etc., 

 of the Cobalt series, resting in an ancient valley of the 

 Keewatin series, between Cobalt and Sasaginaga lakes. A 

 reverse fault — normal to the line of section — occurs parallel 

 to the longer axis of Cobalt lake, and it is also found at the 

 McKinley-Darragh about one-quarter of a mile to the south- 

 west, and at La Rose at the north end of the lake. 



6—7 



