GEOLOGY AND PALEONTOLOGY. 143 
GENERAL CHARACTER OF STRATIGRAPHY. 
To the cursory observer traveling through central Kansas or along the 
‘‘Bench road’? from Tekamah to Decatur, Neb., it might appear that sandstone 
ledges occupied the greater part of the Dakota group. The most conspicuous 
objects are bold and rugged shoulders, buttresses, and cliffs of massive sandstone. 
This sandstone is of all shades of color, from gray to black, with brownish red 
predominating. The ledges may, in fact, be traced for miles, forming the escarp- 
ment of a ridge or outcropping as a definite ledge along the side of a bluff. 
Such geologists as Hayden and Capellini seemingly failed to recognize the fact, 
or at least did not record it, that the greater part of the Dakota group is not 
sandstone, but consists of sandstones and shales. At a conservative estimate, 
at least two-thirds of the rock of the group is not sandstone. It is true that in 
many places they are strongly arenaceous, but not enough so to constitute them 
sandstones. 
The shales and clays of the Dakota vary much both in texture and color. 
Not infrequently they are black and papyraceous, reminding one very much of 
the Kiowa. Usually, however, they are white, blue, or yellow, with many bands 
of red or green. A Dakota clay bank ordinarily presents a variety of colors. 
The darker shales are usually more highly argillaceous; while the lighter colors, 
such as yellow and gray, indicate a greater amount of sand. Another fact that 
should not be allowed to escape notice in a systematic study of the group is the 
non-continuity of both sandstones and shales. There are almost no persistent 
ledges of hard rock with shales between, as is the case in either the Permian or 
Benton. For example, the Cottonwood limestone, near the top of the Carbon- 
iferous, is persistent for over 200 miles; and in the Benton the ‘‘fence-post 
limestone’? may be traced perhaps half as far. Nothing like this occurs in 
the Dakota. It is true that in a few instances pronounced ledges may be traced 
across one or two counties. As examples may be cited the ledge along the 
Missouri river below Sioux City ; also the ledge which caps the Smoky Hill buttes 
and Soldier Cap mound, in Saline county, Kansas, and forms the escarpment 
along Spring creek, above Brookville, as far as the Ellsworth county line. (See 
plate VIII.) Even these ledges, however, which are exceptions to the general 
rule, vary much in thickness. 
Throughout the group the sandstone is not constant in lithological qualities. 
It represents all degrees of hardness, from very soft sand, which may be crushed 
with the fingers, to the hard, brittle, clay-ironstone concretions, which turn the 
edge of the hardest steel. A bank of argillaceous shale will oftentimes become 
arenaceous, change to soft sandstone, and this, in turn, become hard enough to 
form conspicuous ledges; all in less than 100 yards. Not infrequently these 
changes may be observed several times in the course of a few miles, the ledges 
appearing and disappearing with no apparent regularity. 
Massive and conspicuous ledges are not uncommon. Doctor Hayden assigns 
to the ledge above Blackbird mission the thickness of from sixty to eighty feet. 
At Tekamah, Neb., it is over fifty feet from the bed of Tekamah creek to the top 
of the railroad cut; and the entire thickness is through the coarse, dark brown 
sandstone. At the Santee caves, on the Platte, a sixty-five foot ledge is exposed. 
On Mulberry creek, in eastern Ellsworth county, Kansas, a mile east of the 
famous cave section, a ledge measures seventy feet; and at the mouth of Alum 
creek, ten miles south, is one nearly as thick. These, however, are exceptions. 
Ten feet would, perhaps, be a fair average for the thickness of Dakota ledges. 
The strata of clays and shales are usually much thicker; but because of the 
fact that they are composed of softer material are more easily denuded. A 
