COAL MEASURES. 131 



under black shale beds, and is underlaid by light-colored shales, 

 and fire-clay containing Stigmaria. Lately a number of experi- 

 mental borings have been made in the vicinity of Chester. The 

 coal formation is found considerably eroded and partially swept off 

 during the drift period. The shales and inclosed coal seams are 

 generally deprived of the protecting roof of sand rock and lie close 

 under the drift, which is a very unfavorable condition for mining. 

 The coal seams appear also to be often replaced by hard black 

 bituminous shales. Of one of the borings at Chester, I received 

 the following record : Drift, 8 feet ; hard black shales, slate-like, and 

 inclosing thin seams of coal, from 6 to 8 feet ; whitish, fine-grained 

 sand rock, containing Stigmaria, Lepidodendron and Calamites, 7 

 feet ; whitish plastic fire-clay, 30 feet ; black shales with pyrites, 30 

 feet; white fire-clay with hard, ferruginous bands, found at the 

 bottom of the bore-hole. 



The most instructive natural section through the coal formations 

 which we have in the State, is seen at Grand Ledge, in the valley 

 of Grand River, 10 miles below Lansing. The river has carved 

 its bed there to a depth of about 60 feet below the general surface 

 level of the country. The upper part of the hills bordering the 

 valley is formed of drift ; the lower presents a section through the 

 rock beds of the coal measures. The village of Grand Ledge is 

 located nearly in the centre of the outcrops, which continue up 

 and down the river for about a mile. The strata rise and sink 

 in undulations, which bring the higher and lower beds to repeated 

 outcrops on the same level. The order of stratification, often 

 visible in sections of large horizontal extent, gives a fair opportu- 

 nity for observing the changes to which a stratum in its horizontal 

 extension is often subject with regard to thickness and quality of 

 material. The observed variability explains why, in the numerous 

 sections seen within the limited space of a few miles, no one ex- 

 actly corresponds with the other, although many of them represent 

 about the same horizon. 



The upper part of the formation is a coarse-grained sand rock from 

 25 to 30 feet in thickness. In the locality where I saw it best ex- 

 posed, the rock occupies one of the depressed curves of an undula- 

 tion such as has been alluded to, and at both ends of the exposure 

 lower rock strata come up alongside the upper beds on the same 

 level. The sand-rock ledges form a compact body, with only insig- 



