JUNCTION OF SUPERIOR SANDSTONE AND KEWEENAW AN TRAPS \) 



Thickness of Sandstone Series 



Where the sandstone has been thrown into an anticline of considerable dimen- 

 sions, as already mentioned, an opportunity is had to measure the thickness of 

 this series. (In passing, it may be of interest to note that in geological work done 

 some years ago there was at times a tendency to mistake secondary cleavage for 

 bedding, and thus to make possible enormously large estimates of the thickness of 

 certain formations. At this locality, however, the opposite and very uncommon 

 mistake was made, and bedding and accompanying cleavage were regarded as 

 purely secondary, while a series of horizontal joints were clearly thought to be 

 bedding planes.*) On the southern limb of this anticline, which is about 3,000 

 feet across and along which exposures are pretty continuous, the dip averages 

 approximately 70 degrees toward the south. This gives a thickness, as exposed, 

 of about 2,160 feet. It is not unreasonable to suggest that the total thickness of 

 the sandstone at this locality does not greatly exceed this figure, for the upper beds 

 are marked slides, which in places are highly charged with lime, thus possibly 

 representing the beginning of the physical changes which brought on the epoch 

 of the Lower Magnesian limestone, a formation which overlies the Lake Superior 

 sandstone in the northern peninsula of Michigan, but which is not known in the 

 western part of the Lake Superior basin ; and the lowest beds exposed along the 

 axis of the syncline are quite coarse, and even conglomeratic, thus indicating a 

 possible approach to the base of the series. This estimate of the thickness of the 

 Lake Superior sandstone is regarded as conservative ; still it shows that the forma- 

 tion is of considerably greater vertical dimensions than has been commonly sup- 

 posed.! 



Amount of Displacement 



In regard to the amount of this displacement data are not complete ; but it is 

 very probable that the throw increases from west to east, and the amount of dis- 

 placement on the east, where the sandstone has been thrown into the anticline 

 just noted, equals at least the thickness of the exposed strata, or over 2,100 feet. 

 How much more it is impossible to state, although to this figure we can add some- 

 thing for the lower unexposed strata of the sandstone, and to this at least the 

 present vertical distance to which the traps near the junction rise above the sand- 

 stone. A total vertical displacement of some 2,500 feet is thus quite clear, and not 

 improbably the distance is greater than this.J 



Professor Grant's paper was discussed by C. R. Van Hise and A. C. 

 Lawson. 



*Geol. of Wis., vol. iii, p. 347, 1880. 



fFrom a well-boring at Ashland, Wisconsin, the thickness of the Lake Superior sandstone has 

 been stated to be over 2,500 feet (G. L. Collie, this Bulletin, vol. 12, p. 200, 1901). The writer is not 

 aware that any detailed study of the borings from this well has been made. Such a study might 

 show that some of the sandstone penetrated had the Iithological characters of the Upper Ke- 

 weenawan sandstone, which is here thought to underlie the Lake Superior sandstone at an unde- 

 termined depth. 



X Since this paper was written and presented another paper has appeared (C. W. Hall, this Bulle- 

 ' tin, vol. 12, pp. 313-342, 1901), in which is presented evidence for the extension of this fault plane 

 for a considerable distance to the southwest in Minnesota. 



II— Bull. Geol. Soc. Am., Vol. 13, 1901 



