THE GEOLOGICAL SECTION OF MICHIGAN 425 



concentration and after all chemical erosion and deposit were much 

 more important in the Grand Rapids than at any time since the 

 Traverse. 



32. Maxville or Bay port limestone, Upper Grand Rapids, Upper 

 St. Louis, Middle Kaskaskia. (50 to 235 feet usually eroded).— 

 This formation marks the culmination of a transgression.' Generally 

 it is only 50 to 75 feet thick or less, and seems to be much eroded 

 away by a heavy erosion and uplift that took place after its formation. 

 But in the Mount Pleasant well 235 feet may belong here. 



Light, hard limestones, bluish with chert and white sandstones 

 are characteristic. It is the typical old subcarboniferous limestone. 

 Faunally (with Allorisma, Lithostrotion canadense, etc.) it is also 

 closely alhed with the Upper St. Louis, the middle of the Kaskaskia, 

 and the Ohio Maxville — an epoch of maximum ocean extent at this 

 time. I do not know any good reason for not calling it Maxville.^ 

 Owing to the heavy subsequent erosion there is no telling how far 

 it may have extended, but it certainly extended into Huron and 

 Arenac counties and thence west. It also extended south of Jackson, 

 and may once have gone into Ohio continuously. 



However, in a region from Tuscola County south around Durand, 

 Morrice, and Howell, there seems to be an area where it does not now 

 occur and perhaps never occurred. An anticlinal uplift either 

 prevented its formation or caused it to be eroded away. From Jack- 

 son to Grand Rapids past Bellevue and Assyria, however, there 

 are frequent signs of its presence though the coal measures are laid 

 upon it with a very marked unconformity. 



On the whole, the climate was not one that favored the formation 

 of shales, but limestone, «chert, and clean white sandstone rather, 

 and as the continent was sinking the rivers tended to aggrade and 

 leave their mud before reaching the sea. 



At the close of the Bayport formation the state was quite likely 

 lifted entirely above water for quite a while,-^ since wells in the center 

 of the basin as near to each other as Alma, Mt. Pleasant, and Midland 

 show very different sections, and the Parma conglomerate base of 



1 Schuchert's Tennessee which he makes early Chester, PI. 81. 



2 See Michigan Miner, December, 1906; U.S.G.S. Water-Supply Papers, 182 83. 



3 Schuchert, PI. 82. 



