conflicting Views of Lake Superior Stratigraphy. 127 



Irving* inclined to the belief tbat such structural breaks as that 

 described in the Marquette district are of great extent, and 

 this accords with the general trend of modern structural work. 

 From what has gone before it appears exceedingly probable 

 that the structural break between the UpjDer and Lower Mar- 

 quette is identical with that which separates, even in a more 

 pronounced manner, the Animikie and Kaministiquia Series 

 and the Upper and Lower Yermilion Lake Series on the other 

 side of the Lake Superior basin. This break, being thus so 

 strongly marked at points so far separated, would argue that it 

 extends over a very considerable area of the Lake Superior 

 region, not improbably from the most distant rock-series before 

 mentioned, the Quartzites of Dakota and the Original Huro- 

 nian of the north shore of Lake Huron. It could not be ex- 

 pected that a like succession would be found in each of the 

 areas parallelized, even if they all belong to the same geolog- 

 ical series. In the first place, the rocks in some districts are 

 not sufficiently tilted to make it certain that all of the layers 

 are exposed. Farther, nine-tenths or more of the surface of 

 the country over large areas is heavily covered by the drift, so 

 that it is all but certain that formations which exist at the rock 

 surface have not been discovered. Still farther, no satisfactory 

 explanation has yet been made of the subordinate succession of 

 formations in the Marquette, Felch Mountain, Menominee and 

 Yermilion Lake districts. So it is not yet known how far the 

 order which prevails in one of the districts is equivalent with 

 that which prevails in another. From recent work it is proba- 

 ble that future investigations will show that this likeness is 

 greater in the series below correlated than has been suspected. 

 But even supposing the discordances are so great as the present 

 known facts might lead one to suppose, it would not be any 

 very strong evidence against the correlations ; for it is not to 

 be expected that the same conditions of sedimentation would 

 prevail at all times in a geological basin 800 miles in diameter. 

 While in one part of the basin fragmental sediments were accu- 

 mulating, it would be very strange if it were not the case that 

 chemical sediments or organic sediments were elsewhere accu 

 mulating. Below it is shown that the Penokee and Animikie 

 Series are the geological equivalents of each other in the broad- 

 est sense of the term. It is not necessarily true that sedimen- 

 tation began or ended simultaneously in both regions, but only 

 that in the main they stand as time equivalents. How far a 

 correspondence can be made out among the subordinate mem- 

 bers of the various districts can only be determined by a de- 

 tailed investigation of each of the areas. 



* On the Classification of the Early Cambrian and Pre-Cambrian Formations, 

 R. D. Irving, U. S. G-eol. Survey, Seventh Ann. Rept, p. 391. 



