SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 269 



They include a quartzite, believed by Todd to be the Sioux quartz- 

 ite, granite, and mica schist. 



F. T. Thwaites^ concludes that the sandstones of the Wisconsin 

 coast of Lake Superior form a single conformable series, which may 

 be separated into two groups, which grade into each other, viz., 

 a lower group, consisting of steeply tilted red feldspathic sandstones 

 and fragments of igneous rocks, shales, and conglomerates; and an 

 upper group, composed of slightly disturbed red and white quartz 

 sandstones. The contact of the upper group with the Middle 

 Keweenawan traps is marked by a thrust fault. A small amount of 

 conglomerate in the sandstones along this contact may indicate a 

 local unconformity. Since the faulting, folding, and erosion of the 

 traps and the deposition of the sands went on simultaneously, there 

 is no reason for believing that this conglomerate represents a great 

 time interval, if indeed any. Irving placed the upper group in the 

 Cambrian since he regarded the sandstone faulted against the 

 Middle Keweenawan traps at St. Croix Falls and the same in age as 

 the sandstone at the falls of the Amnicon River. Thwaites shows 

 that the former is Upper Cambrian, but that the latter is older. 



The conditions of deposition of the sandstones appear to have 

 been dominantly subaerial as indicated by current marks, mud 

 cracks, rain prints, irregular and curved bedding, depressions in 

 shale beds filled with sands, red color, and feldspathic content. 



The relations of these sandstones to the known Cambrian of 

 Wisconsin is still doubtful. Thwaites points out certain differences 

 between them. The Cambrian is marine as shown by fossils, and 

 its sands generally consist of rounded, yellow quartz grains often 

 enlarged so as to show crystal faces, and cemented dominantly with 

 calcite. The sandstones of the Lake Superior coast lack fossils 

 and were largely deposited under subaerial conditions. Their 

 feldspathic content, angular grain, irregular quartz enlargements, 

 red color, irregular and curved bedding, and lack of calcite cement 

 contrasts strikingly with the characteristics of the known Cambrian. 



Van Hise and Leith's^ "The Geology of the Lake Superior 

 Region" is a summary of the geology of the Lake Superior region. 



' F. T. Thwaites, "Sandstones of the Wisconsin Coast of Lake Superior," Wis. 

 Geo!, and Nat. Hist. Surv. Bull. 25, 191 2, pp. 109. 



^ "The Geology of the Lake Superior Region," U.S. Geological Survey, Monograph 

 52, 1911, 641 pp., 49 pis., 76 figs. 



