walcott.] SUMMARY MISSOURI. 339 



conformably against eroded clifls of Keweenawan rook. At St. Croix Falls light- 

 colored Potsdam sandstone containing characteristic fossils reposes unconformably 

 upon and against Keweenawan cliffs and occupies depressions and valleys formed 

 through its erosion. Conglomerates derived from it mark the junction of the two 

 formations and contain Potsdam fossils. Hand specimens may be obtained, having 

 Keweenawan rock for one side and Potsdam sandstone containing shells of Lingulepis 

 pinnaformis for the other. The phenomena absolutely forbid any explanation based 

 on faulting or intrusion 



North of St. Croix Falls the same strata, sustaining a like stratigraphical relation- 

 ship (though not seen in actual unconformable contact) may be traced more than half 

 way across the Keweenawan series. Passing the remainder of the interval to Lake 

 Superior the horizontal red sandstones are found abutting, in similar unconformable 

 contact, against the eroded Keweenawan series as above stated. The accompanying 

 section illustrates the general relationship, but only a careful study of the details 

 .can make clear the full force of the evidence. 



When to these considerations there are added others less susceptible of brief state- 

 ment, to which we are here confined, it appears that the distinctness of the horizontal 

 Lake Superior sandstone from that of the Keweenawan system and the correctness of 

 its reference to the Potsdam series is sustained by a weight of evidence that would not 

 be seriously questioned but for complications with what wo deem the misinterpreta- 

 tions of other geological features of the Lake Superior region. The modifications 

 which the formation assumes in that region are precisely those which its method of 

 derivation demands. 1 



CANADIAN EXTENSION. 



On the geological map of the Dominion of Canada, published in 

 1882, the Nepigon series of rocks is colored Cambrian, both on the 

 shores of Lake Superior and about Lake Nepigon. This is in accord- 

 ance with the views of Dr. A. E. C. Selwyn. 



MISSOURI. 



The Cambrian rocks in Missouri occur in the southwestern portion of 

 the State, about the Ozark Uplift. As far as known they are of Upper 

 Cambrian age and consist of a sandstone that occurs beneath the third 

 magnesian limestone, or Calciferous, and the fourth magnesian lime- 

 stone of the Missouri survey, beneath which, according to Prof. G. C. 

 Broadhead, there are other arenaceous and calcareous beds. 



The third sandstone is doubtfully identified by Mr. F. B. Meek, in 

 Miller County, as C feet of white sandstone, composed of rounded grains 

 of quartz cemented in part by calcareous matter. Subjacent to this 

 there are 27 feet of hard gray and light flesh-colored magnesian lime- 

 stones in rugged, irregular beds from 5 to 8 feet in thickness. 2 In 

 Morgan County the third sandstone is known to occur at a few locali- 

 ties, where it varies from 25 to 30 feet in thickness. The subjacent fourth 

 magnesian limestone forms a bluff 150 feet high above the Osage Eiver 

 in the southwestern part of the county. 3 



1 General geology. Historical geology. Paleozoic era. Geology of Wisconsin, survey of 1873-1879, 

 vol. 1, 1883, pp. 122-124. 

 'Reports on the Geological Survey of the State of Missouri, 1855-1871, 1873, p. 137. 

 »0p. cit., p. 148, 



