crane: " horsebacks" in the KANSAS COAI. MEASURES. 15I 



Strata including the coal beds. The great variety of fissures as 

 above described corresponds well with different forms of fissures 

 observed in many parts of the world in connection with the mining 

 for metalliferous deposits. Upon the production of such a fissure 

 the great pressure under which the fire clay at the bottom of the 

 coal had been existing would now be relieved on one or more sides. 

 If the fissure passed entirely through the fire clay the surface of 

 each wall would be relieved of pressure; if it only reached down- 

 wards to the fire clay the upper surface would likewise be relieved 

 of pressure. Considering the exceedingly unctuous property of 

 the clay and the softening to which it would be subjected from 

 time to time by the underground water, it is very easily under- 

 stood how it would soon move upwards sufficiently to more or less 

 completely fill the fissure produced by the earth's tremors. This 

 process would simply be an exaggerated case of ordinary "creep- 

 ing " so commonly noted in underground workings, the upturning 

 of the shale laminae near tlie upper part of the fissure would very 

 readily be produced by the upward movement of the clay acting" 

 under the great power which was forcing it along, while the occa- 

 sional fragments of coal and sandy shale found within the "clay 

 veins" can readily be accounted for by the occasional dropping of 

 a block which was almost broken under the first earth movements 

 which produced the fissures. 



