53 s THE JOURNAL OF GEOLOGY. 



interlaminations of limestone with shale or sandstone at the transition horizon 

 which frequently occurs when a limestone formation rests upon a sandstone 

 formation. 



As to the age of the Keweenawan, this series is placed by_Dr. Wadsworth 

 as a lower part of the Potsdam, but is regarded by the reviewer as resting 

 unconformably below the Potsdam, and as belonging to a different geological 

 period. This question is one of great complexity, which can not here 

 be discussed in detail. However, Dr. Wadsworth refers the Keweenawan 

 so doubtfully to the Potsdam that the difference can hardly be said to be a serious 

 one. The statement that the most probable explanation of all the phenomena 

 at Keweenaw Point is that the Eastern sandstone is of different ages can 

 have but one meaning — that a part of this so-called Eastern sandstone 

 belongs to the Potsdam, and this Potsdam is later than, and unconformably 

 upon, the Keweenaw series, which latter includes another part of the Eastern 

 sandstone. Put in another way, Dr. Wadsworth extends the term Eastern 

 sandstone to cover all of the sandstone exposed until the Traps are reached. 

 That is, the break between the Potsdam and Keweenawan is in places a short 

 distance away from the Traps. This admits the difference in geological age 

 between the main area of Potsdam sandstone and the Keweenawan, and 

 merely shifts the boundary line between the two a short distance. It is nota- 

 ble that the most important new evidence presented upon the question is that 

 obtained by Mr. Seaman, Dr. Wadsworth's assistant. Near the South Range 

 he finds outcrops which he regards as Eastern sandstone, holding indurated 

 fragments derived from adjacent ledges of upper Keweenawan sandstones, 

 and hence believes the Eastern sandstone to represent a later geological age. 



It appears to the writer very doubtful whether the large number of mem- 

 bers given for the Republic and Holyoke series will be found to be gen- 

 eral for the Lower Huronian and Upper Huronian on the south shore of Lake 

 Sueprior, although each may be found at some locality. 



Wadsworth 1 states that recent work renders it probable that the Azoic or 

 Archean of Northern Michigan is divisible into five unconformable forma- 

 tions. The tentative arrangement, commencing with the oldest, with the par- 

 allel formations, as determined by the United States Geological Survey, is as 

 follows : 



Michigan Geol. Survey. U. S. Geol. Survey. 



Cascade Formation. Fundamental Complex. 



Republic Formation ) 



Mesnard Formation \ Lower Marquette series. 



Holyoke Formation [ 



Negaunee Formation \ Upper Marquette series. 



1 Subdivisions of the Azoic or Archean in Northern Michigan," by M. E. Wadsworth. 

 In Am. Jour, of Sci., vol. xlv., No. 265, Jan., 1893, PP- 7 2 > 73- 



