THE GEOLOGICAL SECTION OF MICHIGAN 419 



Clinton limestones, but back into black shales, like the base of the 

 Antrim, the Berea, or Sunbury shale. 



I am inclined to believe there was originally less sulphate in the 

 brine and that would point to a less arid cliniate,' and with that the 

 black shales and greater abundance of mud are in agreement. At 

 any rate the Berea Grit seems to mark an episode apart from any 

 great climatic change. 



29. Coldwater shale. Part of old Waverly; Sunbury or Berea 

 shale, plus Cuyahoga (which includes Buena Vista), plus Raccoon 

 and part of Black Hand, perhaps; Orangevilie and Sharp ville ? 

 1,000 feet. — The next series lithologically corresponds to the New 

 York Portage and Chemung and is one largely of shales, which gen- 

 erally make valleys in the bed-rock surface and in a state so heavily 

 drift laden as Michigan are rarely exposed. In Ohio there are two 

 or three distinctions clearly made. The thickest, most carefully 

 studied, and best exposed section is that of Huron County/ as 

 follows : 



Blue and sandy shales of Willow River and Sees. 2 and 3, Huron 

 Township 172 



Black Hand of Ohio ? in part 



Light House Point conglomerate, Herrick's I, Large fauna 4 176 



Directly under should come the Raccoon, Herrick's Waverly 

 shale fauna. 



Blue shales with carbonates of iron of Port Hope, Harbor Beach, 

 White Rock to Forestville, with Chonetes scitulus, cf. pulchella, 

 common throughout 720 896 



Black Sunbury shale with Lingula melie and Oroiculoidra newberryi 



in Ohio , 103 999 



practically an even 1,000 feet 



The black shale base is very persistent when the Berea Grit comes 

 beneath, and continues as at Alma and Grayling, even beyond its 

 limits and is presumably the equivalent of the Sunbury or Berea 

 black shale of Ohio. Like that it is a persistent and widespread 

 horizon whose thickness is generally only 25 to 55 feet, averaging 

 about 40. It is clear that in well records there must be some uncer- 

 tainty as to whether we are dealing with the red Bedford or the red 



1 See analyses in Clarke's data of geochemistry. 



2 Vol. VII, Geol. Surv. of Mich., Part 2, pp. 18-27, 247-52, PL I. 



