HISTORICAL SKETCH— LAKE SUPERIOR REGION. II5 



conglomerates, so well described by Lawson, grade more or less 

 rapidly on the one hand into the schists, and on the other into 

 the solid gneissoid granite. The complete change may occur 

 within a short distance, or it may take a mile or more. 



The Basement Complex is then composed of intricately inter- 

 locking areas of granitoid rocks and schistose rocks. Moreover, 

 all of these rocks are completely crystalline. None of them 

 show any unmistakable evidence of having been derived from 

 sedimentaries, but many can be traced with gradations into 

 massive rocks, and therefore the greater proportion of them are 

 igneous, if a completely massive granular structure be proof of 

 such an origin. 



The Basement Complex is the most widespread of any of the 

 Lake Superior systems, and it doubtless runs under all later for- 

 mations to a greater or lesser distance. That it is continuous un- 

 der all such formations can not be asserted, for while it was once 

 so, it is possible, perhaps even probable, that in places, as a con- 

 sequence of sedimentation and folding, the Basement Complex 

 has been so deeply buried, that fusion has locally resulted. It 

 is even possible that such fused material is a partial source of the 

 later volcanic eruptions. 



Before the earliest sedimentary rocks were deposited, the 

 Basement Complex was subjected to enormous orographic forces, 

 which folded and sheared the rocks in a most intricate manner. 

 Accompanying the great orographic movements, which undoubt- 

 edly occupied a vast period of time, were intrusions of various 

 deep seated igneous rocks, and also doubtless their volcanic 

 equivalents were extruded. Subsequent to, and during the oro- 

 graphic movements, atmospheric forces were at work. Erosion 

 continued long after the mountain -making folding had ceased, 

 and, for much of the Lake Superior region, reduced the Base- 

 ment Complex nearly to a plain or base level. As evidence of 

 this mav be cited the fact that, at the end of the erosion inter- 

 val, the Basement Complex, consisting of differing lithological 

 materials, and therefore having a variable resisting power, did 

 not varv in altitude more than a few hundred feet for lons^ dis- 



