19 



deepest hollows are, however, scarcely 20 feet lower than the tops of the 

 hillocks. Generally these changes of consistence of the strata arc marked by 

 mere undulating prairies. A few cases of the interstratifications of materials 

 apparently differing in composition, are mentioned in remarking upon the 

 localities where they have been observed. The general character of the group 

 may therefore be described as a succession of more or less coarse, sandy 

 materials, regularly stratified, more or less impregnated with oxide of iron, 

 and, according to the prevalence of this mineral, either hard, compact, dark 

 red, or yellowish, composed of sandy grains more loosely cemented and more 

 easily disintegrated ; the whole mass being interlaid by deposits of fine pot- 

 ter's clay, yellowish, white, or red spotted, or black, rarely mixed with car- 

 bonaceous matter, distributed more generally near the base and the top of the 

 formations in areas of small extent. 



The thickness of the Dakota group has not been as yet, and could 

 scarcely be, exactly ascertained without borings made at many distant localities. 

 In the land of prairies where it prevails, the rivers are bordered by banks whose 

 elevations are from 30 to 60 feet, rarely reaching an altitude of more than 100 

 feet. In South Nebraska, around Beatrice, the section made by Dr. Hayden 

 in his report, (1867, p. 27,) estimates the whole thickness of the group at 

 150 feet. It is the same measure which I marked in ascending from the 

 quarries on the Big Blue, counting it from the upper exposure of the Permian, 

 the shaly fossiliferous limestone, to the top of the highest hill above, composed 

 of red shale in its whole thickness. But at a short distance to the south there 

 is still a succession of hills of the same formation which are at least 40 feet 

 higher. On the banks of the Missouri River, near the Indian reservation, the 

 bluffs of sandstone, as exposed above the alluvial deposits which cover their 

 base, measure from 70 to 100 feet, generally perpendicular. I have seen, 

 probably about at the same localities where Dr. Hayden made the section 

 recorded in the report of 1867, 1 the sandstone exposed 120 feet from its base, 

 where it is apparently covered by 20 to 25 feet of detritus and alluvial matter, 

 and from the top, in ascending along a quarry road, I dug here and there from 

 the surface loose pieces of the same sandstone, to 70 feet higher. According 

 to Meek and Hayden, the dip of the strata of the whole series, which rests 

 conformably upon the Permian limestone, is to the northwest. 2 This dip is of 



1 Geological Survey of the Territories, p. 46. 



2 American Journal of Sciences and Arts, vol. xxvii, 1859, p. 35. 



