JUNCTION OF SUPERIOR SANDSTONE AND KEWEENAWAN TRAPS 7 



Superior geology. In brief these conclusions are as follows : The junction be- 

 tween the Eastern sandstone and the Keweenawan traps is along a fault. The 

 displacement in the traps began before the deposition of the sandstone. The sand- 

 stone was deposited along a shore cliff formed by the fault scarp. Since the deposi- 

 tion of the sandstone further displacement lias occurred along this earlier line of 

 faulting. Thus the apparently inferior position of the sandstone is due, not to the 

 fact that the sandstone is older than and underlies the traps, but to faulting in 

 part subsequent to its deposition ; the Keweenawan traps belong to the upper 

 division (Algonkian) of the pre-Cambrain, while the sandstone belongs to the 

 upper half of the Cambrian. The two formations are thus separated by an uncon- 

 formity. 



Comparison of South and North Side of Lake Superior Syncmne 



All this is on the south edge of the Lake Superior syncline. When the junction 

 of the Lake Superior sandstone and the Keweenawan traps along the north side 

 of this syncline is considered, it is found to be a marked coincidence that the rela- 

 tions of the two formations, at least as far as faulting is concerned, are very similar 

 to those already described. Mr E. T. Sweet,* on the former Geological Survey of 

 Wisconsin, collected data along this line of junction, and from this data Professors 

 Chamberlinf and Irving j inferred that a fault existed here. While employed by 

 the present Geological and Natural History Survey of Wisconsin, the writer had 

 occasion to study this junction of the sandstone and traps, and to obtain informa- 

 tion which demonstrates conclusively, even more conclusively than on Keweenaw 

 point, the faulting and the later age of the sandstone. $ 



Features of Contact 

 contact line 



This contact line of the two series has an east and west direction in Douglas 

 county, Wisconsin, being in general parallel to the lake shore and lying only a few 

 miles from it. The most important phenomena presented may be classed under 

 three heads : topography, effect on the traps, effect on the sandstone. 



TOPOGRAPHY 



In passing southward from the west end of the lake one crosses a monotonous 

 plain which abuts against an east and west ridge known as the Douglas copper 

 range. The north slope of this ridge is abrupt, and its summit is 100 to 300 feet 

 above the plain. The plain is underlain by the Lake Superior sandstone in hori- 

 zontal beds, and the ridge consists of lava flows dipping steeply toward the south. 

 The sandstone in general is not firmly cemented and is easily eroded, while the 

 traps or lava flows are much more resistant to erosion. The abrupt northward 

 slope of the trap ridge functions as a fault scarp, although by this statement it is 

 not intended to convey the idea that the faulting has been recent, nor that erosion 

 has not kept pace with it; on the other hand, the present scarp is regarded as 

 a result of differential erosion. 



* Geol. of Wisconsin, vol. iii, pp. 340-350, 1880. 



f Geol. of Wisconsin, vol. i, p. 105, 1883. 



% U. S. Geol. Survey, Monograph v, p. 258, 1883. 



§ Wisconsin Geol. and Nat. Hist. Survey, Bulletin vi, 1900. 



