STEATIGRAPHY OF PLA:NrT-BEAEING TERRANES. 5 



boniferous occuvred during a period of terrestrial subsidence and advance 

 of the shore line,' the result of which is the theoretically complete conceal- 

 ment of the earliest beds of the Coal Measures beneath the landward over- 

 laps of the succeeding- sediments. The maximum thickness of the lower 

 concealed beds is difficult to estimate, since in the borings farther out 

 toward the interior of the basin the upper beds are probabl}'' thinner and 

 the means for the identification of the individual terranes of the section are 

 more or less unsatisfactory. In one instance Dr. Keyes^ observed a body 

 of shales not less than 75 feet in thickness occupying a concealed ravine in 

 the Mississippian series. The nature and extent of the suljjacent terranes 

 lying farther out in the basin can be calculated only from the borings or 

 from the analogies furnished by the series in other sections in which the 

 horizons of the lower coals may be approximately ascertained by the study 

 of the paleontologic evidence. 



In the region of Henry County, from which most of the material 

 under examination was obtained, the loose surface detritus of the eroded 

 Mississippian is generally covered by an extremely variable sandstone, 

 described in various reports as the "Ferruginous sandstone," "Spring River 

 sandstone," etc., and generally correlated by the Missouri geologists with 

 the "Millstone grit," though its representative in Illinois was regarded 

 by the geologists of that State as a part of the Eocarboniferous. This 

 sandstone, the age of which, so far as I can learn, has not yet been deter- 

 mined from any paleontologic evidence, is never of great thickness, and 

 is described as here and there more or less eroded. It serves largely as a 

 leveling medium, tending to fill the ravines and hollows of the Mississip- 

 pian, with whose loose cherty subaerial detritus it appears to be somewhat 

 blended. At some points it is reported as entirely wanting, having perhaps 

 been eroded prior to the sedimentation of the coals and sandstones. 



Resting either immediately on the somewhat uneven surface of tliis 

 "Ferruginous sandstone," or in places perhaps directly on the Mississippian 

 detritus, lie the shales, sandstones, limestones, and coals of the Lower Coal 

 Measures, which as originally defined were stated to have a thickness of 

 about 250 feet,^ including the " Ferruginous sandstone." All the plant 



1 Bull. Geol. Soc. Am., vol. iii, pp. 283-310. Am. Geologist, vol. xii, p. 102. 



^Broadhead, Rept. Geol. Surv. Mo., 1872, pt. 2, p. 6. Winslow, Bull. Geol. Soc. Am., vol. iii, 

 1892, p. 115. 



