424 A. C. LANE 



thick — the fullest of any in the state. There is always a gypsum 

 or anhydrite bed near the middle of the formation, and with this is 

 associated dark-colored dolomites and dark-blue shales. Sandstones 

 are thin and irregular and in some cases there are dark limestones 

 toward the base. These dark, impure limestones, are quite different 

 from the Bay port limestone. The section around the margin if at 

 all full is something like 200 feet, but from Tuscola County south to 

 the Assyria are a lot of wells in which it is hard to recognize this or 

 the Bayport at all. Occasionally, as around Byron, very salt water 

 near the surface may indicate outliers of it. The water from the 

 formation is salty and "bitter," full of calcium and magnesium 

 sulphate and in that respect very different from the Marshall immedi- 

 ately underlying. The absence of the Michigan series from the 

 southeastern part of the state seems to be due not wholly to erosion 

 at top, but to uplift of the bottom, the emergence of the Marshall 

 having progressed so far that this part of the state like the corre- 

 sponding part of Ohio was out of water. 



The date of this emergence during which the Lower Michigan 

 was forming in the center of the basin is pretty definitely fixed on 

 paleogeographic grounds as that of the Upper Augusts or Osage.' 

 The Michigan series seems to have continued forming until a depres- 

 sion to the west opened connection with the wide ocean at the time 

 of the Maxville of Ohio, Upper St. Louis or Kaskaskia and Chester 

 of the Mississippi Valley. The section seems to be continuous with- 

 out disconformity to the overlying limestone, which I have called 

 Upper Grand Rapids, since both sets of beds were well exposed near 

 Grand Rapids and seem in many ways bound together. The Lower 

 Grand Rapids must then include the Lower St. Louis and probably 

 the Keokuk and perhaps in the center strata representing part of the 

 Burlington, the time Kinderhook-St. Louis including an era of emer- 

 gency in which all of Michigan but a central sea was out of water. 



The dark and sometimes even black slates and the blue and dark, 

 impure dolomites give the formation a more muddy look than the 

 Salina, while the general association of dolomite and gypsum is like 

 that of the Salina, and one is inclined to believe that some land waste 

 and rain erosion were still going on, though local conditions favored 



I The emergence between Schuchert's Mississippi and Tennessee. 



