182 



GEOLOGY 



truncated before the deposition of the next succeeding system, show 

 that thousands of feet of sedimentary rock must have been removed 

 from their crests. Such remnants of this formation as escaped destruc- 

 tion are in some places known to possess a thickness of about 6000 

 feet (Marquette district of northern Michigan), and if all the so-called 

 Lftronian of the area north of Lake Huron be Huronian, within the 

 meaning of the term as here used, the thickness is 1 8,060 (?) feet. 1 In 

 other places the Huronian strata were completely removed, together 

 with an unknown thickness of the Orphean below. According to 

 such data as are at command* touching this question, it does not seem 

 improbable that the erosion interval which followed the uplift of the 

 Huronian beds was as protracted as the interval during which they 

 accumulated. 



iJAKi-1 



; The erosion of a land surface implies sedimentation about its bor- 

 ders. The materials worn from the Archean and Huronian formations 

 during this period of erosion were probably carried to the sea in tne 

 main, and deposited chiefly near its shores; but where the shores were 

 during this time there are no means of knowing, since the deposits then 

 made seem to have been covered, in large measure, by later sediments, 

 and not since exposed.tr r nn 



Contemporaneous events elsewhere. — The processes which were 

 going on about Lake Superior were doubtless in operation about every 

 other land-mass of the time; but it does not necessarily follow that 

 the areas of sedimentation were extended and contracted in other 

 regions just as they were about Lake Superior. It is a principle gener- 

 ally observed in geological nomenclature that the name applied to a 

 series or a formation in one place is not given to a series or a formation 

 in another, unless there is strong reason for believing the two to have been 

 contemporaneous, or essentially contemporaneous, in origin. While, 

 therefore, there are rocks in other localities which doubtless correspond 

 in age with the Huronjari of Lake Superior, their contemporaneity has not 

 been proved, and the term Huronian is not generally applied to them. 2 



1 Ante, p. 151. 

 f 2 For a fuller description of the "Lower" Huronian of the Lake Superior region, 

 and of the early Algonkian of other regions, see Van Hise, Bull. 86, 16th Ann. Rept. 

 Monos. XIX, XXVIII, XLIII, and XL, and the 21st Ann. Rept., Pt. Ill, U. S. Geol. Surv. 

 For a discussion of the principles involved in the study of complex systems of non- 

 fossiliferous rocks, such as the Archean and Algonkian of Lake Superior, see, in addi- 

 tion, Irving, 7th Ann. Rept., U. S. Geol. Surv. 



