536 



A. G. LEONARD 



beds of lignite (Fig. 8). These vary in thickness from an inch 

 and less to thirty-five feet, beds six, eight, and ten feet thick 

 being common (Fig. 9). Coal is much more liable to be present 

 in the Fort Union than in the underlying Lance formation, for the 

 latter is practically barren of coal in many localities and over 

 large areas. One rarely finds an outcrop of the former where 

 several hundred feet of strata are exposed that does not contain 



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Fig. 11. — A burning coal bed. The surface over the coal has settled many feet 

 and the ground is broken by wide cracks from which gases escape. Typical Fort 

 Union beds in background. 



at least one or more coal beds. These range from top to bottom 

 of the formation and do not appear to be confined to any particular 

 horizon or horizons. The aggregate thickness of the twenty-one 

 coal beds of southwestern North Dakota which are four feet and 

 over is 157 feet. 



One of the most conspicuous features of the Fort Union is the 

 vast quantity of burned clay or clinker produced by the heat of 

 the burning coal beds (Fig. 10). This has been sufficient to burn 



