crane: " HORSEHACKS " IN THE KANSAS COAI, MEASURES. 



149- 



liaving the iron, and perhaps other constituents, leached out from 

 the shales and adjacent coal. In a few instances the clay filling 

 contains fragments of coal or of sandstone similar to that which 

 may be seen in the shales above, implying that the coal and sand- 

 stone fragments have fallen into the fissure while the clay was 

 accumulating. 



The fire clay is very finely divided, and as a general rule has no 

 regular structure; yet in some cases a lenticular structure is noted, 

 the convex surfaces of the lens-shaped masses being nearly hor- 

 izontal. The color of the fire clay is generally light, varying from 

 a light yellow to pale bluish or soapstone color. The general 

 appearance of the matrix indicates that at one time it has been in 

 a plastic, semi-liquid, homogeneous mass from which the excess 

 of water has gradually drained away. Before exposure to the air 

 this clay is usually quite hard and tenacious, presenting a formid- 

 able obstacle to the miner. But on being exposed to the air it dis- 

 integrates and assumes more of the common properties of clay. 

 Where the ''clay veins" or "horsebacks" are abundant the 

 "room and pillar" S3^stem of mining is usually employed and the 

 masses of clay are used to as great a degree as possible to support 

 the roof. 



Fig .'). Horseback made uj) of two varieties of fli-e-ciay. 



