SUBDIVISIONS OF COAL MEASURES. 



79 



layer, the Keokuk and the upper and lower Burlington lime- 

 stones. 



The Kinderhook group is a three-fold division whose sev- 

 eral members are strongly contrasted and persistent over wide 

 areas. 



Upper Carboniferous Series or Coal Measures. 

 The Coal Measures of the upper Mississippi region rest in 

 marked unconformity upon the strata beneath. Within most 

 of the district the rocks immediately below are portions of the 

 Lower Carboniferous, but sometimes older formations come in, 

 Devonian or even Silurian. The surface upon which the coal- 



Fig. 8. Coal Measure Sandstone Resting on Saint Louis Limestone. Keokuk. 



bearing strata is laid down is everywhere uneven, the irregu- 

 larities being of such a nature as to leave but little doubt that 

 it is an old land surface. The character of this old erosion 

 plane is shown in the accompanying fisrure (8), and also in 

 plate ix. 



Over the greater part of the Carboniferous area of the 

 Mississippi basin, the Coal Measures are capable of being sepa- 

 rated into two portions — one, which forms a marginal zone and 

 is commonly called the lower division, and the other, deposited 

 in more open water, termed the upper section. The two for- 

 mations are usually tolerably well contrasted in a general way, 



