22 IER JKOUIANAIL (Qh? (GIS OWL OG MW, 
The calcareous passages are four in number, and each exhibits 
a rhythm of sedimentation. The lowest occurs 210 feet above 
the base of the Benton group, and exhibits an alternation of thin 
limestone beds with somewhat thicker beds of shale, the shale 
being more calcareous than the general mass. Each limestone 
is a few inches in thickness, and the intervening shales are from 
one to two and one-half feet thick. The average thickness of a 
pair of beds, including a limestone layer and a shale layer, is 
eighteen inches, and the number of such repetitions is about 
fifteen. 
Next above are 230 feet of shale, and upon these rest about 
fifty feet of limestone, constituting the basal member of the 
Niobrara group. These limestones alternate in an equally regular 
manner with shales, the layers of limestone being homogeneous 
and massive, and varying in thickness from one foot to two feet, 
with three feet as a rare and local maximum. The parting shales 
range from one to four inches in thickness, and are sharply sepa- 
rated from the limestone. The average thickness of the rhythmic 
couple, limestone and shale, is as before, eighteen inches. 
The third calcareous series, also of Niobrara age, lies ninety feet 
above the second, the interval being occupied by shale. Through 
a thickness of thirty feet calcareous shales alternate with those 
which are less calcareous, and the amount of calcareous matter 
increases upward, culminating at the top of the series in two 
beds of chalky limestone. The average rhythmic interval here 
is between two and one-half and three feet, and the differences 
in rock texture are of such character as to give a ribbed appear- 
ance to the series where exposed on a cliff face. 
The fourth calcareous passage is at the top of the Niobrara 
group, and is separated from the third by 475 feet of shale. It 
includes several calcareous layers, of which one might be classed 
as an impure limestone. The rhythmic tendency is clearly mani- 
fested, but the number of repetitions is small. 
The Pierre shales, overlying these, have a thickness of about 
2800 feet. 
From these data it appears that, in addition to a secular and 
