STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 161 



tensive variation in coal beds. Two beds appear to be really per- 

 sistent for long distances; he had traced for 120 miles one which 

 he regards as the equivalent of a bed in North Dakota. The coal is 

 lignitic throughout, though it often resembles sub-bituminous. After 

 weathering, the grain of the wood disappears, the color changes to 

 black and the material is no longer tough, but is brittle. The greater 

 part was formed from trunks of trees and fragments of wood ; even 

 entire logs, usually prostrate, can be traced on the fresh face of a 

 mine. Coal in the lower 500 feet of the formation is less woody in 

 appearance than that from the upper 500 feet. The extreme vari- 

 ability of the coal beds led him to infer that conditions were very un- 

 stable in the old moors. Beckly found the lignite very tough and 

 wood-like in the Culberston field. 



In considering the remarks on ^Montana areas, one must bear in 

 mind that in most of the region the studies have been confined to 

 natural outcrops and that tracing of the beds has been made in con- 

 siderable areas by means of clinker lines, the burned outcrops. Ex- 

 tensive mining operations are concentrated, the localities being very 

 few. The intervals between coal beds are reported as varying 

 greatly. Speaking in a general w^ay, it would seem that the measure- 

 ments are too few for determining whether or not such variations 

 are merely irregularities. The comparatively few detailed measure- 

 ments are not enough to show the relations of the several benches 

 of any bed in a large area. There is enough, however, to raise doubt 

 respecting the actual continuity of the beds for any considerable 

 distance. 



Leonard,--' in his synoptical description of the Dakota region, 

 calls especial attention to the great variability of the accompanying 

 rocks. The coal seams are from one inch to 33 feet thick and usu- 

 ally they are not persistent in extended areas. A seam may be 

 pinched out or perhaps it may be replaced by another at the same or 

 a slightly difi'erent horizon. Two seams may overlap, so that while 

 both are to be seen in one section, only one of them may be present 

 at half a mile away. Some can be traced in the river bluflfs for 



--' A. G. Leonard, " North Dakota-Montana Lignite Area," U. S. Geol. 

 Survey, Bull. 285, 1906, pp. 316-330; A. G. Leonard and C. D. Smith, "The 

 Sentinel Butte Lignite Field," Bull. 341, Pt. 2, 1907, pp. 15-35. 



