678 W. O. HOTCHKISS THE LAKE SUPERIOR GEOSYNCLINE 



During the deposition of these sediments there was probably pro- 

 gressive sinking due to the cooling and contracting of that part of the 

 original magma that was not extruded. The cooling and solidifAdng of 

 this mass Avould result in giving off large quantities, of solutions and 

 gases, and the contraction would result in settlement and readjustment 

 of the lava flows above it to make channels for the escape of the solutions. 

 It is suggested that this igneous after-effect may be appealed to as the 

 source of the copper mineralization which the members of the geological 

 department of the Calumet and Hecla Mining Company have presented 

 to us. 



After the deposition of the Upper Keweenawan sediments orogenic 

 pressure from the southeast resulted in tilting the sediments and increas- 

 ing the northward dip of the south limb of the syncline. This stress 

 resulted in a great thrust-fault across northern Douglas County, in Wis- 

 consin, which throws the middle or lower part of the flow series in contact 

 with the youngest Upper Keweenawan sediments known. 



The succeeding history of the Lake Superior syncline has been one of 

 gentle elevations and sinkings in all following geologic periods, in which 

 the structure has not been altered, but has acted as a unit. The only 

 modification has been that due to erosion, which has developed the 

 surface as we see it today. 



