I 1 8 LO WER PENINS ULA . 



Gray lime 4 ft. 



Sandy fire-clay mixed with seams of harder rock 5 1 " 



Soft sand rock 37 " 



Hard gray limestone 2 " 



Soft white sand rock 15 *' 



Blue limestone i " 



White fire-clay i " 



Sand rock 4 " 



Fire-clay with iron pyrites 50 " 



Soft sand rock 5 " 



Blue limestone 16 " 6 in. 



In deep borings made at Flint, no rocks similar to the car- 

 boniferous limestones have been found, neither does the record of 

 Blackmar's salt well, south of Bridgeport, say any thing of lime- 

 stone beds perforated ; but in nearly all the boring records of 

 Saginaw and Bay City, the horizon of the carboniferous limestones 

 is plainly discernible. In the Saginaw Bay district, we find fine, 

 natural exposures of the carboniferous limestones, northward on 

 the branches of Rifle and Aux Grees rivers. 



If we follow Aux Grees River from the point where I have de- 

 scribed the gypsum beds (in Town. 21, R. 5, Sect. 20), upward to Sect. 

 30 of the same township, we find the creek running over arenaceous 

 limestone ledges, and in its banks about 12 feet of limestone 

 strata are seen above the water-line. The lower arenaceous lime- 

 stones are interstratified with dark, blackish shales. One of the lower 

 beds is also pervaded by the Syringopora-like ramifications of 

 cherty substance which I have described in the limestones found 

 north of Jackson, on Portage River ; here also no organic structure 

 can be discovered. The upper portions of the ledges in the out- 

 crop are light-colored, smooth-fracturing limestones, containing 

 many hornstone concretions, and of fossils, an abundance of Allo- 

 risma clavata, with other species, such as Productus Flemmingii, Sy- 

 ringopora ramulosa, Zaphrentis spinulosa, and various forms of 

 Fenestella. A chain of outcrops of these limestones extends be- 

 tween the parallel streams. Rifle and Aux Grees, to Point Aux 

 Grees. This point is a low land-tongue of not over 10 feet eleva- 

 tion above the lake level. On both sides for the distance of a mile 

 it is lined with low cliffs of calcareous sand-rock ledges, with flint 



