COMPARATIVE STUDY OF LIMESTONES. 123 



pact and approaches a shaly character. But while so strikingly distinct 

 from the Echo lake limestone, we are finally impressed with the suspicion 

 that the Bruce limestone is the same formation, but deposited nearer the 

 shore, or, at least, in less quiet waters.* 



Conclusion. 



Two Systems of Eoch in the Hiironian Region. — Now, since the position of 

 the Echo lake limestone is between the upper and lower slate conglomerates, 

 the Ansonia and Ottertail limestone must hold a similar position — that is, 

 it must be in the position of Logan's " e. Limestone;" and hence Logan's 

 "^, Yellow chert and limestone" is the same thing, and has no separate 

 existence, as indicated by him. In a table, however, which we have pub- 

 lished, '=^ we placed the Ottertail limestone above the upper slate conglom- 

 erate, thus, though on independent evidence, agreeing with Sir William 

 Logan ; but it is probable we both have been in error. If, then, things are 

 so, the geographical position of the Ottertail limestone remains to be ex- 

 plained, as both slate conglomerates lie on the same side of it ; and we may 

 need to invoke a fault to bring it up from a position 9,500 feet below. We 

 may still be in ignorance of a limestone actually seen between the two slate 

 conglomerates as occurring in the Thessalon region. But this ignorance is 

 not fatal, for it may result either from insufficient observation or from a local 

 absence of the intervening limestone. 



But the adjustment of the complicated geology of the Thessalon valley is 

 not necessary to the establishment of our main contention. We have shown 

 that in the original Huronian region there are two systems of strata, petro- 

 graphically dissimilar and structurally unconformable, the characteristic 

 part of one being the upper slate conglomerate, and of the other the lower 

 slate conglomerate. We have shown that the same two systems exist on 

 Thunder bay and in northeastern Minnesota, as also, apparently, in the 

 Marquette, Gogebic and Penokee regions. We have identified the upper 

 system on Thunder bay and in Minnesota with the upper system north of 

 Lake Huron. We have identified the lower system on Thunder bay and in 

 Minnesota with the lower system north of Lake Huron. We have shown 

 that if any doubt could exist respecting the distinctness of the two systems 

 north of Lake Huron, none can be entertained (as none has been enter- 

 tained for many years) respecting their distinctness on Thunder bay and in 

 Minnesota. 



But both these systems were included in the Huronian, and are still in- 

 cluded in it on the north shore of Lake Huron. Both were also included 

 at first in the Huronian of Thunder bay, but since 1848 the upper has been 

 excluded, and the name Huronian has been applied only to the lower; while, 



* Sixteenth Ann. Rep. Minn., pp. 569, 570. 



