GEOLOGY AND PALEONTOLOGY. 141 
fixed line be drawn in order sharply to define the limits of the various formations. 
This, however, it seems difficult to do. 
It should be remembered that Doctor Hayden, who named and described the 
group, did not at that time know of the existence of older Cretaceous rocks in the 
west; but supposed that the Dakota was the basal member of the Cretaceous 
and rested in all places directly upon the Carboniferous or Permian. It is the 
presence of Lower Cretaceous formations immediately beneath the Dakota that 
seemingly complicates the problem. In regard to the upper limits of the group, 
Doctor Hayden frankly admits that there is great difficulty in drawing the line 
between the Dakota and the Benton, because of the fact that both the stratigraphy 
and fossils of the two groups blend in numerous localities. In speaking of the 
exposures near Sioux City, he says: ‘‘ Lithologically, it is impossible to draw the 
line between these formations here. No. 1 (Dakota) passes so imperceptibly 
into No. 2 (Benton), and No. 2 into 3 (Niobrara), that there is no break, and yet 
their principal characteristics are very distinct. The first is a sandstone, the 
second a black, plastic clay, and the third a chalky limestone; and yet I cannot 
tell the exact point where one commences and the other ends.” 
If this be true for the line between the Dakota and the Benton, what shall be 
said of the base of the Dakota, where conditions are much more complicated? 
As far as the observations of the writer go, it is rarely difficult, in any one of 
the dozens of localities he has visited in Kansas, Nebraska, and Iowa, to decide 
upon a line of separation, at least approximately, between the Dakota and the 
Benton. The sandstones of the former group gradually thin out, yellowish 
arenaceous shales become prominent, these become darker and more argillaceous, 
and are succeeded, at a distance of from fifty to eighty feet above the last sand- 
stone, by the typical Znoceramus limestone so characteristic of the Benton. 
The conditions at the point of separation of the Comanche and Dakota are 
quite different. In the localities where the best exposures of rocks of both these 
series occur, Belvidere, and Clark county, Kansas, the upper member of the 
Comanche, known as the Kiowa shale, consists of about 150 feet of dark, papyra- 
ceous shale, usually becoming arenaceous above. This shale contains a charac- 
teristic marine fauna, both vertebrate and invertebrate, which is frequently less 
abundant in the upper layers. Before the fossils disappear, the shales are, in 
places, superseded by ledges of sandstone more or less massive, and as much as 
fifty feet thick. In other places no fossils are found above the shales. The 
sandstone ledges become more pronounced and soon begin to contain a dicoty- 
ledonous flora. 
In central Kansas similar conditions obtain. As has been stated, in this 
region the rocks which occupy the lower part of the Cretaceous groups were first 
classed as Triassic, then Jurassic, then Dakota, then called ‘‘Mentor beds’’; 
and are now recognized as representing the northern limit of the Comanche or 
Belviderean sea. Paleontologically, these measures differ but little from the 
Kiowa of southern Kansas. Stratigraphically, the rock is very like the Dakota, 
or, rather, it combines the characters of the Kiowa and Dakota. In a section 
taken near Marquette, Kan., which is probably typical as regards stratigraphy, 
there are twenty-nine feet of sandstone and seventy-six feet of shale.” The 
shale predominates in the lower part of the section, where it is very like the 
Kiowa. Above, sandstones occupy the greater part of the thickness. The upper 
stratum in the section is a heavy ledge of rather hard, dark-brown sandstone, 
which contains an abundant Mentor fauna, of which Doctor Stanton has recog- 
nized twenty-two forms. Fifty feet below this ledge is a layer of soft sandstone 
47. American Geologist, 25:35. 
