52 GEOLOGY OF ILLINOIS. 



bedded limestones at the base, passing upward into green and bluish colored 

 shales, which are overlaid by ferruginous sandstone, the latter representing the 

 base of the Coal Measures. On Waters's Branch, a half mile south of this 

 mill, there is a fine exposure of the regularly bedded limestone of this group, 

 about ten feet thick, forming a perpendicular wall along the banks of the creek. 

 There is a bed of earthy gray limestone about four feet thick, intercalated in 

 it at this point, that appears like a hydraulic rock. The concretionary mem- 

 ber of this group, outcrops on the upper course of McGee's creek, three miles 

 southeast of Columbus, and with the regularly bedded limestones below, con- 

 tinues along the bluffs of this creek, through its whole course in this county. 

 In the vicinity of Hughes's ford, on section 27, township 2 south, range 5 west, 

 the brown magnesian limestone of this series is well exposed, the bed ranging 

 from ten to fifteen feet in thickness. It is about thirty feet above the bed of 

 the creek, and overlies the geodiferous shales of the Keokuk group, which 

 extend below the creek level. In the Coatsburg coal shaft, this limestone was 

 reached at a depth of about one hundred and forty-seven feet, and the shaft 

 was carried on through it, and into the geodiferous shales of the Keokuk 

 group, where it terminated at a depth of about two hundred feet. On the 

 Walnut fork of Mill creek, about four miles a little south of west from Colum- 

 bus, this limestone is exposed on the southeast quarter of section 21, township 

 1 south, range 7 west, and as it is only about seven miles to its outcrop on Mc- 

 Gee's creek, east of that town, it is probable that it constitutes the bed rock 

 entirely across the divide between these points, and separates the coal, south 

 of Columbus, from that in the north part of the county. In the vicinity of 

 Mendon, this limestone was met with at several points, and is overlaid by the 

 coarse qu.artzose sandstone of the Coal Measures. Here the upper part of it is 

 a light gray, more or less concretionary rock, from ten to twelve feet in thick- 

 ness, below which, we find the brown magnesian limestone, and the shaly beds, 

 which form the lower division of the group. This limestone is also found well 

 exposed on the tributaries of Bear creek, in township 2 north, range 8 west, 

 and on the main creek, on its upper course, for some distance further east, 

 where it passes beneath the Coal Measures, and the latter becomes the bed rock 

 over all the northeastern portion of the county. 



This limestone may be readily distinguished from any of the lower divisions 

 of the Lower Carboniferous series, either by its lithological characters or the 

 fossils which it contains. The light grey concretionary limestone, is charac- 

 terized by two species of fossil corals, one or both of which may be found at 

 nearly every locality where the rock is exposed, and are often met with in fine 

 specimens, weathered out of the limestone, and lying in detached masses in the 

 debris along the streams. They are generally silicious, and where they have 

 not been rolled and water-worn after being detached from the rock, they retain 



