43 



plastic shale, the mobility of the shale particles would permit 

 this zone to adjust the inequalities of strain resulting from 

 the unequal contraction in the coal seam. Such conditions 

 exist in the roof of coal number 6 in the Carterville-Zeigler 

 district of southern Illinois. Rock rolls occupying depressions 

 in the top of the coal are here common, but no clay seams pene- 

 trate the coal bed. In the vicinity of these rolls the roof shale 

 is cut by slickensided zones for a distance of several feet from 

 the center of the roll, indicating a considerable lateral move- 

 ment in the shale in accomplishing the adjustment of the strains. 

 However, the roof of coal number 5 is a hard, brittle shale 

 whose constituent particles do not possess the mobility requisite 

 for such adjustment. If the limestone cap rock was very thick 

 it might be able to withstand, without fracture, the unequal 

 strain due to unequal contraction in the underlying coal seam. 

 But the cap rock of this coal is thin, averaging only twelve to 

 fourteen inches. The combined strength of the roof shale and 

 cap rock was not sufficient to withstand the unequal strain to 

 which they were subjected, and Assuring of the beds resulted. 



Immediately above the cap rock of this coal seam is a bed 

 of rather soft, gray shale or soapstone whose particles were 

 sufficiently mobile to bring about adjustment in the unequal 

 strains which, by the Assuring of the roof shale and cap rock, 

 had been transferred to this higher horizon. The materials 

 from this shale horizon were immediately squeezed downward 

 through the fissures as wedges into the coal seam, until the in- 

 equality of pressure was adjusted. Under these conditions the 

 place in which adjustment was accomplished was limited to a 

 narrow zone below the point where the fracture was made in 

 the roof shale and cap rock. Hence the effects were confined to 

 a narrow zone horizontally, but they became thus strongly 

 marked in a vertical direction. 



It is probable that from time to time, as the shrinking in 

 the coal mass continued, more qlay was forced downward into 

 the coal seam, fissuring it still deeper and spreading the walls 

 of the fissure constantly wider apart. The abundant evidences 

 that the clay filling the fissures in the Springfield coal seam was 



