PART I. 

 INTHODUOTIOlSr 



THE LIGNITIC FORMATIONS OF NORTH AMERICA. 



§ 1. — Areal distribution. 



The country west of the Missouri, and to the base of tlie Rocky 

 Mountains, is for nearly six hundred miles — as from Omaha to Cheyenne, 

 or from Kansas City to Denver — a vast plain, with a gradual slope, unappre- 

 ciable to the eyes, and without any of those land irregularities which gen- 

 erally, breaking the stratification by upheavals or denudations, expose to view 

 the rocks composing the crust of the land surface. The ascending grade 

 from the Missouri River toward the mountains does not average more than 

 ten feet per mile, and as the Cretaceous strata exposed below Omaha above 

 the Permian Measures, are nearly horizontal, they pass, of course, toward 

 the west under the different stages of the Tertiary or under more recent 

 deposits. The great uniformity of the plains and the absence of exposed 

 rocks prevent the distinct tracing of the line of demarkation between the 

 Cretaceous and the Tertiary. As far as it is known in the States of Iowa, 

 Kansas, and Nebraska, the average width of the belt occupied by the Dakota 

 group, which is in that country the lowest member of this formation, is from 

 sixty to one hundred miles.* Over this appear the Upper Cretaceous groups, 

 which, where they have been observed along the Missouri River, have a 

 thickness of more than two thousand feet, or, for the whole Cretaceous forma- 

 tion, two thousand five hundred feet. As from Omaha to Cheyenne, which 



* Eeport of the United States Geological Survey of the Territories, by Dr. F. V. Ilaydeu, vol. vi, 

 Cretaceous Flora, p. 12. 



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