IOWA ACADEMY OF SCIENCES. 129 



have modified but slightly the topographic forms of its sur- 

 face, adding only the last touches to the features which the 

 region presents to-day. 



A POSSIBLE ORIGIN FOR THE LIGNITES OF NORTH 



DAKOTA. 



BY FRANK A. WILDER. 



The lignite of North Dakota occupies part of an area 

 that is shared by the neighboring states of Montana, Wyo- 

 ming, and South Dakota, 'and by Assiniboia on the 

 north. The total area of this lignite field in the United 

 States alone is 70,000 square miles, apportioned as follows: 



North Dakota 31,500 



Montana 25,000 



Wyoming 9,000 



South Dakota 4,500 



It is probable that all of this field does not contain lig- 

 nite beds of workable thickness, but studies carried on in 

 North Dakota and Montana during the past summer indi- 

 cate that thick beds are very general, and that often a 

 series of four or five are separated by relatively thin strata 

 of clay. Some of the beds are very thick, one which out- 

 crops near the Little Missouri in southwestern North 

 Dakota measuring forty feet, while twenty-foot beds are 

 not uncommon. 



Analyses of samples taken from more than sixty points 

 in North Dakota show that, except for the high percent- 

 age of moisture that they contain, they might be ranked 

 as semi-bituminous coal, since the amount of fixed carbon 

 is unusually high for lignite. The average North Dakota 

 lignite contains thirty-two per cent of moisture. When 

 this is driven off by heating to 100 C, analyses show that 

 the average lignite is composed of: 



