426 A. C. LANE 



the coal measures appears sometimes at one level, sometimes at 

 another. The Bayport is apparently entirely gone at Alma. 



This, then, would be the line between the Mississippian and Penn- 

 sylvanian, in this state the strongest disconformity since that at the 

 base of the Devonian, and the first time that there is any evidence that 

 the whole of the state was above water. 



PENNSYLVANIAN 



33. Parma conglomerate; Pottsville (170 feet; basal mem- 

 ber). — The recurrence of deposits in Michigan is marked by a bed 

 of conglomerate. The pebbles are not always present, to be sure 

 and are rather small and very white, about like split peas, and the 

 mass of the formation is sandstone. The name is taken from a point 

 on the margin of the basin which is very likely contemporary with 

 shales, etc., in the center. As a term, then, it is, like the Potsdam, 

 not to be taken as of definite age but as the underlying basemental 

 and shoreward facies of the Saginaw formation. As a very persistent 

 horizon easily recognized by the presence of pebbles, which are rare 

 in the Michigan column, and as an economically important water 

 bearer it deserves a place in the column. Compared with the 

 Marshall brine beneath, it has less of the earthy chlorides, more of 

 the sulphates. 



The wells of the Saginaw Plate Glass Works yielded a set of sam- 

 ples which show the characteristic Parma and the strata above and 

 below. 



34. Saginaw formation. Upper Pottsville (400). — This is the 

 coal-bearing series of Michigan. All the other formations seem 

 actually to dip and occur deeper at the center than at the margin of 

 the basin. Mr. Barnes, chief driller of the Consolidated Coal Co., 

 thinks that for this, too, the marginal coal seams, at Sebewaing and 

 Jackson, correspond to the deepest seam at the middle. I am 

 hardly inclined to think it. Their chemical character is more like 

 upper seams and the fauna and flora of the upper seams at the center 

 and at the margin seem similar. The series is a succession of white 

 shales (so-called fire clays) or sandstones, black shales (called slate) 

 and coal, and blue shales, with occasional thin bands of black band 

 ore (siderite) , and nodules of the same containing zinc blende and iron 



