38 



THE CLAY SEAMS (HORSEBACKS) IN THE NU3.IBER 5 

 COAL BED, NEAR SPRINGFIELD, ILLINOIS. 



T. E. Savage. 

 The principal coal seam exploited in western Illinois, over 

 the area between Springfield and Peoria, was designated in the 

 ^\"orthen reports as number 5. The following section shows 

 the character of the strata associated with the number 5 coal 

 in the Springfield quadrangle: 



FEET 



Sandstone or shale 10 to 30+ 



Light gray shale or soapstone 1/^ to 4 



Limestone cap rock /<2 to I73 



Black, fissile shale 2^/' to 4^^ 



Coal (Numbers) 5 to 6>4 



Fire clay 1 ^ to 5 



One of the conspicuous features of the number 5 coal seam 

 is the presence of numerous clay-filled fissures that extend down 

 into, or through, the coal seam from the overlying beds. The 

 fissures are generally from two or three to sixteen inches in 

 width, although the larger ones attain a width of three or four 

 feet. Their walls are slickensided, but do not show any traces 

 of weathering. The spaces between the walls are filled with a 

 light gray shale or soapstone. 



These fissures, with their fillings, are known to the miners 

 as horsebacks. There is no regularity in the distance between 

 these horsebacks, or clay seams, or in the direction in which 

 they extend. In some mines they are encountered about forty 

 to sixty feet apart, while in others they are separated by a 

 distance of two hundred to four hundred feet, or more. They 

 traverse the coal scam in various directions, no single direc- 

 tion greatly predominating even in the same mine. 



The shale filling the fissures is light gray in color and is gen- 



