LoGaN. | The Upper Cretaceous. 207 
is in Clay and Cloud counties. In the northern portion the Dakota 
beds rest upon the Permian gypsiferous shales, and in the southern 
upon the Red-Geds of the Permian. 
The Dakota formation may be subdivided, but there are no well 
defined separating lines. For convenience of discussion, however, 
it will be divided into two groups; first the ferruginous or lower 
eroup; and second the saliferous shale group, or upper group. 
The layer of sandstone which underlies the upper shales will be con- 
sidered the line of separation. 
THE LOWER GROUP. 
The lower group consists of alternate layers of red and white 
sandstone, and argillaceous and arenaceous shales. In the south- 
eastern part of the Dakota area these shales rest upon the Permian 
limestone; in the northeastern part upon a thin bed of gypsiferous 
shales which probably belong to the Permian. The shales of the 
lower bed are chiefly argillaceous, but near the sandstone layers 
they are frequently found to be highly arenaceous. In many places, 
especially in the strata adjoining the lignite veins, they are very 
bituminous. They are of many colors, but the color usually prevail- 
ing isa light blue. An outcrop in the Smoky Hill Buttes shows three 
bands of color, red, mottled and white. Small traces of lignite are 
found in the lower bed of shales, as are also thin layers of sandstone. 
Gypsum crystals are found usually near the middle of the bed. The 
maximum thickness of this lowest bed of shales is in the neighbor- 
hood of one hundred feet. The remainder of the lower group con- 
sists of alternate layers of shale and sandstone with the sandstone 
layers far exceeding the shales in thickness. The thickness of each 
layer of sandstone varies, however, in different parts of the Dakota 
area. Near the middle of the sandstone group is a layer from which 
many fossil leaves have been taken, but they are not always found 
at exactly the same horizon, their position varying as much as fifty 
or sixty feet. The best specimens have been collected in the eastern 
part of Ellsworth county, none having been found thus far in the 
northern part of the Dakota area. The sandstones of the ferru- 
ginous group rest on the lower beds. They probably reach their 
maximum thickness in the northern part of the area, and thin out 
