THE GEOLOGICAL SECTION OF MICHIGAN 415 



DEVONIAN 



25. Dundee limestone. 200 to 253 feet. — This formation — the 

 Corniferous or Onondaga nearly — is full of fossils which have been 

 described by Hall, Winchell, Rominger, Schuchert, Grabau, and 

 others. I On the east side of the state between the blue and black 

 shales that may represent the Bell or Marcellus and the first dolomite, 

 which seems to be generally the top of the Silurian (of course, there 

 may be a puzzling dolomite conglomerate at times) the formation 

 can be traced persistently. It is very uniformly a high-grade lime- 

 stone with only a small percentage of magnesia, not infrequently 

 over 98 per cent CaCOj, light colored, or brown with oily matter, 

 containing a water relatively high in sulphates, relatively weak and 

 strong in HjS, and generally hard. It is sometimes, not always, 

 cherty or "corniferous." Beginning with a thickness of an even 100 

 feet in the southeast corner of the state, it thickens slowly to Port 

 Huron. Going west and north it at first thickens until it gets its 

 full thickness of about 250 feet, and then begins to thin, as I now 

 believe. 



For instance, the Niles well on p. 280 of the report for 1903 may 

 be interpreted as having only 12 feet of Dundee and then entering 

 the Monroe, and that in Vol. V as (the Oriskany being at 540 feet) 

 having but 40 feet, and all these wells in the southwest corner as 

 striking through from some part of the Traverse corresponding to 

 the Alpena limestone into the Monroe, the Dundee being omitted, 

 agreeing with Schuchert's map, Plate 75. 



As we go up the Lake Michigan shore northward it is apparent 

 that the Traverse expands to the thick 600-foot section found in its 

 northern outcrop, while the Dundee does not increase so much. 

 The top thirty feet and other places are sometimes quite sandy and 

 often cherty. It is not often sandy on the dividing line between it 

 and the Monroe. 



Throughout my work in Vol. V and the annual reports I have 

 considered all the Dundee as a limestone and this has given consistent 

 results. Four miles east of Mackinaw City, in a section where I thought 

 I found the Dundee directly overlying magnesian limestones of the 

 Monroe, Grabau found in the top layers of the magnesian beds a 



I Annual Report for 1901, and Bull. G.S.A., XVII (1901), 719. • 



