LoGAN. | The Upper Cretaceous. 205 
composed usually of thin bands of sandstone with intervening shale 
beds capped with a heavy thickness of sandstone. See Plates V, 
VI, XU, and XXIV. 
‘he Benton area is more regular in topographic features. It 
contains a single chain of hills which extends from northeast to 
southwest through the central portion of the area, and are composed 
of the upper Benton shales, the Blue Hill shales, capped with the 
I'sort Hays limestone. On account of the peculiar bluish appear- 
ance which they present when seen from a distance, they are called 
the Blue Hiils. Their color is due to the dark blue shales of which 
they are composed. 
The Fort Pierre shales in the northwestern part of the state 
are protected by the “Mortar beds.” Many mounds similar to those 
in the Dakota and Benton areas are found. 
DIP. 
‘ne direction of the dip of the Cretaceous bed is toward the 
northeast. The altitude of the lowest Fort Hays limestone of the 
Niobrara on the Hackberry in Trego county is 2200 feet. The same 
stratum rests upon the Blue hills to the northeast in Mitchell 
county at an elevation of i800 feet. This makes a difference of 400 
feet in elevation for a distance of eighty miles, ora dip of five feet tothe 
mile. A continuation of the same section from Ionia to Scandia pro- 
duced relatively the same result. The altitude of Wallace in Wallace 
county, eighty five miles west of the Hackberry, in Trego county, 
is 3300 feet. Allowing 400 feet for the Niobrara and 100 feet for the 
Fort Pierre at Wallace, we have a difference in altitude of 600 feet, 
making a dip of seven feet per mile. Taking the elevation of 
Otcgo, in Jewell county, 1792 feet, and using the above data for 
Watiace, we find a difference in altitude of nearly 1000 feet in a 
distance of 170 miles, or a dip of six feet to the mile. The eleva- 
tion of Beloit, in Mitchell county, at the extreme Upper Dakota is 
1550 feet. The elevation of Santa Fe, in Haskell county, is 2950 
feet. A well at Santa Fe passes through 324 feet of Tertiary and 
13 feet of Fort Benton, giving an elevation for the upper surface of 
the Dakota at that point of 2611 feet. The difference in elevation 
of the Dakota at Beloit and at Santa Fe is therefore 1261 feet. The 
