THE GEOLOGICAL SECTION OF MICHIGAN 427 



pyrites, and very rarely limestones with marine fossils.' More 

 commonly, but still rarely, in the black shales a Lingula carbonaria 

 or mytiloides, and at Grand Ledge a little pelecypod like Anthracosia 

 occurs, and this Lingula seems to mark a definite horizon, that of 

 the Upper Verne, the two Verne coals often occurring close together 

 and sometimes having a limestone between. 



The fauna and flora indicate Upper Potts ville (Beaver or Kanawha) 

 near Mercer (compare the Kanawha, Black Flint, Mercer limestone 

 and Stockton coal) , and is also near the top of the Saginaw formation. 

 How much lower the base may possibly go we have no means of 

 knowing. But there is reason to believe that there was not continuous 

 deposition even in the center of the basin, and the Upper Pottsville 

 is over 1,200 feet in West Virginia. 



As a whole, the formation is composed of beds of rather rapidly 

 varying thickness and character. This is true also of the coal seams. 

 In one mine they will rise and fall 20 feet and more, pinch out or 

 pass into black shale. A curious feature is a local persistence of 

 facies. That is, in one township there will be a great deal of sand- 

 stone at many levels, at another there will be much shale at all levels, 

 in one region many of the coals will be prominent, in another none. 

 Finding a good upper coal is by no means a sign that the coals below 

 will be extra thin. 



This points to a certain persistence of geographic condition. 

 That is, if a big sand dune or sand bar occurred in a point flanked by 

 a peat swamp on one side and muddy, clay-depositing waters on the 

 other, while it extended more or less widely and shifted a little from 

 time to time, yet it tended to remain in the same general region and 

 even built up as the general level of the water rose. We can see that 

 this might be so by watching the effect of rises and falls in the level 

 of the Great Lakes. A rise of 7 feet may not seriously shift the 

 location of a swamp and the barrier beach that cuts it off from the 

 main lake. 



The writer made a list of some seven coal horizons, to which Cooper 

 has added seven more. When we consider that the whole 14 occur 

 within 400 feet, most of them too thin to work, and that one seam 

 may vary 20 feet or so in elevation in a couple of hundred feet, 



I Vol. Ill, Pa'-t 2, 42, 43, 96, 203; Report for 1907, 19; Report for 1905, 185, 188. 



