468 INDEX TO THE STRATIGKAPHY OF NORTH AMEEICA. 



as the dividing line between two formations in the middle member. Again, future work may- 

 show it possible to divide the beds of PottsviUe age into at least two formations, the upper 

 one including the more shaly portion unconformably overlying the Mansfield sandstone. 



The table opposite shows the subdivisions of the Pennsylvanian that have been made in 

 the several fields of this region, with their known or probable correlations. 



K 16. MICHIGAN. 



The Michigan section of the Pennsylvanian is discussed by Lane^^® in a paper 

 recently published. It comprises the Parma sandstone and Saginaw formation and 

 is wholly of PottsviUe age. 



The Parma is a sandstone with gmall white pebbles of the size of split peas. 

 According to Lane it may be regarded as the basal shore phase which was deposited 

 earlier in the deeper and later in the marginal portions of the basin. It is thus syn- 

 chronous in part with the Saginaw. The thickness assigned to the Parma in a well 

 at Saginaw is 60 feet. 



The Saginaw formation is described by Lane ^"^ as a succession of white shales 

 or sandstones, black shales and coal, and blue shales, with a few thin bands of black- 

 band iron ore and nodules of siderite containing zinc blende and iron pyrites. Lime- 

 stones with marine fossils occur but are rare. Lingula carbonaria or mytiloides is 

 found in the black shales. It appears to mark the horizon of the Upper Verne coal, 

 which in places lies close above the Lower Verne coal, separated from it by a limestone. 



The flora of the Verne coals, which Lane lists, is assigned by David White to the 

 horizon of the Mercer coal, or upper PottsviUe. Nothing younger than PottsviUe is 

 known, and it is probable that the earliest PottsvUle is not represented in Michigan, 

 even in the center of the basin. 



To the account of the coal measures for this volume G. H. Ashley has contributed 

 the following notes regarding Michigan: 



The Pennsylvanian of Michigan is nearly everywhere deeply buried beneath glacial drift, so 

 that our knowledge of its thickness, distribution, and stratigraphy depends almost altogether on 

 drillings. Including the Parma sandstone at the base, it covers an area of about 11,000 square 

 miles, of which the coal-bearing part of the series covers possibly 6,500 square miles.'"* This 

 area is situated in about the center of the Southern Peninsula. The structure is that of a "very 

 flat, gentle syncHnorium, whose longest axis is perhaps from northeast to southwest. This is 

 modified by minor undulations."'^'* The Pennsylvanian has a thicloaess of 700 feet or more, 

 including the Parma sandstone at the base. This sandstone is about 100 feet thick, though 

 measurements up to 145 feet are reported and measurements below 100 feet are conmion. In 

 places the sandstone is absent.^" 



For the rocks above the Parma sandstone the name Saginaw formation has been used.^" 

 This formation consists of sandstone and shale, with coal, clay, limestone, and iron carbonate in 

 nodules or thin beds. Coal occurs at twelve or more horizons. ^'^* The coals appear to lie on 

 floors of irregular surface, varying in thickness to correspond — that is, the coal is thicker in the 

 low places or ' ' swamps ' ' and thinner in the higher places . Many of the low places have the shape 

 of a trough. The coals may range in thickness up to 5 feet; most of the merchantable coal is less 

 than 4 feet, and some less than 2 feet. Under the coal is usually clay, and over it black shale, with 

 blue shale and limestone above. Cannel coal or cannel-hke coal is associated with many of the 

 Michigan coal beds, usuaUy at the top of the bed. Sandstones are abundant in the Saginaw for- 

 mation, many of them as thick beds of white sandstone, in places grading into red at the top.'" 

 They do not occur at any constant horizon in the formation. 



A comparison of the dip of the higher members of the Saginaw with that of the underlying 

 strata and the stratigraphic sections in various parts of the basin indicates that at least part of 



