COAL MEASURES. I 23 



the northern end of Gladwin County, from which point the forma- 

 tion is again known through actual outcrops ; it enters the north- 

 west corner of Bay County, and extends in a southeast direction 

 toward the mouth of Rifle River, striking the shore of Saginaw 

 Bay. The rim of the formation is a few miles north of the Rifle 

 River valley. 



The first discovery of coal in Michigan was made in the vicinity 

 of Jackson, situated near the southern rim of the coal field. The 

 formation is there, in various places, naturally exposed, and is 

 found, in digging cellars or wells within the city limits, under the 

 cover of only a few feet of drift. 



A coarse-grained, whitish sandstone is the highest of the rock 

 series. The east side of Grand River presents various bluffs of this 

 sand rock, in the northern part of the city. Walker's coal mine is 

 located in one of these, and its lately erected buildings are closely 

 attached to the vertical rock walls from which the material for their 

 erection was quarried. A short distance south of the coal mines 

 is the Michigan State Prison, which stands on the sand-rock ledges 

 of the coal measures. Its walls are built of the same material, dug 

 out from the spot, or from others in close proximity to it. 



This upper sand rock has an approximate thickness of 30 feet. 

 By an undulation of the strata, some shale beds below the sand- 

 stone come to the surface in the interval between the State Prison 

 and Walker's coal mine, but this is the greatest depth to which natu- 

 ral sections go ; the deeper beds are only known by artificial bor- 

 ings, of which I will enumerate some, communicated to me by Mr. 

 Walker, and by other parties. The shaft of Walker's coal mine is 

 54 feet deep, commencing in sand rock 26 feet, followed by shale 

 beds with kidney-ore nodules, 17 feet ; bituminous coal, 4 feet ; 

 fire-clay, 7 feet. Below the shaft an excavation to a further depth of 

 30 feet went through nothing but shales and arenaceous rock 

 seams. Total depth of excavation, 84 feet. 



Half a mile east of Walker's mine is Porter and Hubert's mine, 

 the shaft of which, 64 feet deep, commences in drift 1 1 feet in 

 thickness, followed by, 



Sandstone, coarse, whitish . 30 ft. 



Shale, dark . 17" 



Coal 4 " 



Fire-clay opened for several feet. 



