20 



slight degree, indeed ; but considering only the slope of the area covered by the 

 formation in its width from east to west, there is in sixty miles only, as from 

 Papillion Station 1o Schuyler, on the Union Pacific Railroad, a difference of 

 375 feet of altitude. These figures, representing the thickness of the strata, 

 should be still increased by the amount of the dip. It appears, therefore, that 

 the estimate of Professor Swallow of a thickness of about 400 feet for this 

 formation is not overrated. I take this estimate from the report on the geo- 

 logical survey of Kansas, 1 adding to it the three upper members of the fourth 

 section, marked Triassic, overlaying the buff magnesian limestone. The 

 group is described as a compound of brown, ferruginous yellow and buff 

 sandstones, which, says the author, "are generally classed as Cretaceous, though 

 I saw no proof of its age.'' The same thickness is already marked in Hay- 

 den's Report on the Exploration of the Yellowstone and the Missouri Rivers, 

 1859-60, published in 1869, where, in the section of the Cretaceous rocks of 

 Nebraska, the Dakota group is estimated 400 feet thick, as in the section re- 

 corded in the beginning of this chapter. The thickest continuous series of 

 the sandstones which have been measured and recognized as bearing leaves, 

 from the base to the highest point, is that mentioned by Prof. B. F. Mudge, 

 of Manhattan College, in the Kansas Agricultural Report of 1872, where, in 

 Clay County, near Roverdale, Kansas, the fossil leaves were found at the bot- 

 tom of a well 35 feet deep, as low as the bed of the Republican River, and on 

 the top of the adjoining hills, 200 feet high. The thickness of the strata is 

 related, of course, to the depth of the local erosions into the formation which 

 they cover. As indicated from the homogeneity of the compounds, however, 

 it is apparently generally the same, only increasing somewhat toward the west, 

 Having positively recognized the point of superposition of the Dakota 

 group to the Permian limestone, and the permanence in the characters of the 

 strata which come in juxtaposition at the base of this group, it is important 

 also, to look for the same kind of evidence in the succession and modification 

 of its upper strata in the line of union with a higher member of the Creta- 

 ceous. For this we have still to receive our more precise informations from 

 the records of the explorations of Dr. Hayden, who has so carefully stud- 

 ied and so admirably described the Dakota Group, in fixing its limits, and 

 marking its characters, that he has scarcely left anything to discover to those 

 who have followed him in the same field of researches. He has seen at two 



1 Preliminary Report of the Geological Survey of Kansas, I860, p. 9. 



