CARBONIFEROUS LIMESTONE. 1 07 



the limestones of Aux Grees Point ; but a direct contact between 

 the beds is not seen, the interval from White Rock Point to Point 

 Aux Grees being filled by a sand beach, and the country back 

 from the shore being all deeply drift-covered. 



Gypsum beds are frequently penetrated in boring for salt in 

 Saginaw valley, but from these not much is to be learned of the 

 detailed structure of tiie gypsum formation, and the limits of this 

 horizon, either upward or downward, are not clearly recognizable. 

 In some of the bore-holes of Saginaw valley, no gypsum beds 

 were found at all, but the presence of gypsum within the rock 

 beds is perceived in these wells notwithstanding, by the large 

 quantity of gypsum which the water holds in solution. The 

 manufacturers have to carefully close off from their pipes the water 

 streams coming from the gypsiferous horizon, as it will incrust their 

 cavities in a very short time and cause obstructions. In the salt 

 wells of Kawkalin, the gypsum is struck at a depth of 400 feet from 

 the surface. In some wells of Bay City the gypsum horizon is about 

 700 feet below the surface. Further south, in Blackmar's salt well, 

 near Bridgeport, and in the borings at Flint, Lansing, etc., no gyp- 

 sum beds were found. 



West of Alabaster Point, for a distance of 30 miles the gypsum 

 formation can be found near the surface on all the head branches of 

 Aux Grees River. In Town. 21, R. 5, Sect. 20, the shales inclosing 

 gypsum beds are seen in the bed of Aux Grees River and on its 

 banks. Shales of blue or greenish color, interstratified with calcareo- 

 arenaceous seams, inclose large concretionary masses of pure white, 

 rose, or salmon-colored granular gypsum, in quantities which would 

 invite to mining enterprises, but for the fact that gypsum has a 

 limited demand, and that the necessary supply can be much more 

 cheaply quarried in the gypsum beds near the shore, which, from 

 present appearances, are not likely to be exhausted for a great many 

 years to come. Some miles northeast from this latter spot, in Town. 

 21, R. 5, Sect. 12, is another similar exposure of rich gypsum de- 

 posits, and numerous smaller exposures are noticed in the beds of 

 creeks between that locality and the lake. In the central part of the 

 peninsula, along the supposed northern division between the coal 

 field and the Waverly group, no rock exposures whatever are 

 known ; likewise along the same geological belt arching across the 

 south part of the peninsula, no gypsum deposits have ever been dis- 



