196 THE CAMBRIAN. Ibull.81. 



In an article on the age of the copper-bearing rocks of Lake Super- 

 ior, Messrs. Brooks and Pumpelly refer the sandstones of the south 

 shore to the Silurian, and state that they were deposited uuconforina- 

 bly upon the subjacent copper-bearing rocks. 1 



In his report upon the Paleozoic rocks of Michigan, Dr. 0. Romin- 

 ger concludes that the Lake Superior sandstone is the equivalent; of the 

 Potsdam sandstone. He states that there is no record of any instance 

 ill which recognizable fossils were found in situ in the Lake Superior 

 sandstone. 2 The reference of the sandstones of the western side of 

 Keweenaw Point is based upon their lithologic resemblance to those 

 east of the trap range along the center of the point. 



In a paper on some points in the geology of northern Wisconsin 

 Prof. K. D. Irving drew the conclusions that — 



(1) The Copper-Bearing and Huroniau series were once spread out horizontally 

 one over the other and owe their present highly tilted position to one and the same 

 disturbance. (2) That subsequently, after a long period of erosion, the horizontal 

 Silurian sandstones were laid down over and against the upturned edges of the 

 Copper-Bearing series, filling also the synclinal, in Ashland County, which lies be- 

 tween the uorthward and southward dipping sandstones. (3) That hence the Cop- 

 per-Bearing series is more clearly allied to the Archean than to the Silurian rocks. 3 



He describes the series of horizontal sandstones and shales that form 

 the basement rock of the Apostle Islands and the north coast line of 

 Ashland, Bayfield, and Douglas Counties. These he considers in every 

 way to be the equivalent, or, which is more probable, the downward 

 continuation of light-colored Primordial sandstone of the Mississippi 

 Valley. They reappear farther south, on the headwaters of the St. 

 Croix, from whence they can be traced uninterruptedly until they dis- 

 appear beneath the light-colored sandstones of the Mississippi Valley. 4 

 This paper of Prof. Irving's was reprinted in the American Journal of 

 Science, under the title u On the age of the copper- bearing rocks of 

 Lake Superior," etc. 5 



It is stated by Mr. E. T. Sweet that the term " Lake Superior sand- 

 stone" "is generally employed to designate the reddish aluminous 

 sandstones which nearly everywhere border the south shore of Lake 

 Superior. They also form the basement rock of the Apostle Islands. 

 They have never been found in a tilted condition." 6 He considers that 

 Dr. Rominger, of the Michigan Geological Survey, has definitely set- 

 tled that these sandstones are the downward continuation of the light- 

 colored sandstones of the Mississippi Valley; 7 and concludes that the 

 western sandstone of Irving, or that of the Apostle Islands and vicin- 



1 On the age of the copper-bearing rocks of Lake Superior. Am. Jour. Sci., 3d ser., vol. 3, 1872, pp. 

 431-432. 



2 Paleozoic rocks. Geol. Snrv. Michigan, Upper Peninsnla, 1869-1873, vol. 1, pt. 3, 1873, p. 80. 



3 On some points in the geology of northern Wisconsin. Wisconsin Acad. Sci. Trans., vol. 2, 1874, p. 

 117. 



« Op. cit., pp. 114, 115. 



8 Am. Jour. Sci., 3d ser., vol. 8, 1874, pp. 46-56. 



6 Kotes on the geology of northern Wisconsin. Wis. Acad. Sci. Trans., vol, 3, 1876, p. 48. 



7 Op. cit., p. 49. 



