188 BULLETIJ^ 109, UNITED STATES NATIONAL MUSEUM. 



The " argillaceous shales and flags of Lake Huron " are to be iden- 

 tified with the "kidney iron formations." This results from th© 

 identification of the overlying sandstones of the two. Of this for- 

 mation, Mr. Hubbard records the following interesting statements: 



Tliese slates and alteruatiug sandstones may be considered as the upper salt 

 rock of our State. They have been passed through in boring for salt at Grand 

 Kapids, and found to yield strong supplies of brine. At this point they are 

 found also to alternate with beds of gypsum and gypseous marls, as will appear 

 by reference to the table of the strata passed through, given on a subsequent 

 pnge. The thickness ascertained to these slates at that point is about 170 feet. 



An examination of the table referred to shows that the alternating 

 beds of gypsum and shale are the real " Michigan salt group," lying 

 immediately below the lower Carboniferous lime and holding 

 position very far above the " shales and flags of Lake Huron." The 

 identifications by Mr. Hubbard would locate the gypseous shales 

 of Grand Rapids not only below the Point aux Barques sandstones, 

 but below the kidney iron formation ; while the table of borings cited 

 shows them immediately below the limestone which he regards as 

 the bottom rock of the Coal Measures. 



The " soft, light-colored sandstones " are described in the text as 

 "generally of a dark color." Mr. Hubbard states that they have 

 been penetrated in the borings at Grand Eapids, 230 feet; that they 

 " evidently " form the bed of Lake Huron near its foot, but do not 

 make their appearance at any point on the coast of the Peninsula 

 (p. 133). In reality it does not appear evident that they lie on the 

 bed of Lake Huron. This is only an inference from the erroneous 

 identification of the gypseous shales at Grand Rapids with the 

 " shales and flags of Lake Huron." These sandstones, known only 

 under the gypseous shales of Grand Rapids, are identical with the 

 Hillsdale County sandstones. They are the Marshall sandstone, and 

 outcrop as a " soft, light gray sandstone in Ottawa County, near 

 Holland." In Mr. Hubbard's table, therefore, D, F, and H are iden- 

 tical and E and G are identical. 



The " black aluminous slate " is the well-known Genesee shale ; 

 anci the " lim.erocks of Lake Erie " are chiefly the modern " cornif er- 

 ous," but with the " water lime " of the Lower Helderberg exposed in 

 the lowest positions. 



Mr. Hubbard makes statements as follows respecting the dips of 

 the rocks : 



On the enstern slope of the Peninsula, south of Saginaw Bay, northwesterly; 

 while the dip along the southerly and westerly border of the basin of coal- 

 bearing rocks is such as to indicate the counties of Clinton and Gratiot as occu- 

 pying nearly the central part of the coal basin (p. 137). 



