304 The American Geologist. May, 1393 
At Yankton, South Dakota, a short distance above St. Helena 
and on the opposite side of the Missouri the Niobrara beds are 
developed in great force. A large factor}^ has been established 
about three miles west of Yankton to utilize the chalk in the manu- 
facture of Portland cement. The part of the formation at pres- 
ent worked into cement lies above that exposed in the bluflfs at 
St. Helena. It presents a breast about forty feet high. Below 
the base of the present working the chalk is known to descend to 
a depth of about ninety feet. The Fort Benton shales have dis- 
appeared beneath the level of the river; at all events they lie below 
the level of any observed exposures. On the hill tops above the 
■cement factory the chalk of the Niobrara is overlain by the shales 
of the Fort Pierre group. Hayden speaks of this group making 
its appearance on the summit of the hills near the mouth of the 
Niobrara, but he might have found it 30 miles farther east devel- 
oped to a thickness of fifteen or twenty feet. 
The shales of the Fort Pierre group above the chalk, and of the 
Fort Benton group below, are highly charged with crystals of 
selenite, and selenite is b}' no means uncommon in the shaly por- 
tions of the Dakota group near Ponca and Sioux City. 
It only remains to saj^ in conclusion with reference to the tax- 
onom}^ of our Iowa section, that beds one to seven inclusive are 
the stratigraphic equivalents of beds near Ponca, Nebraska, 
which Hayden refers to the Dakota group. Number 8 includes 
beds that at Ponca and St. Helena have been referred to the Fort 
Benton group by the same author, and the luoceramus beds, No. 9, 
are the exact equivalents of the lower twent\^ or thirty feet of the 
Niobrara group. A part of the Jnoceravms beds near Sioux Cit}- 
is soft and chalky; but a part, as has been said, is harder, though 
by no means as hard as ordinary limestone. At St. Helena, 
Nebraska, and, so far as known, at Yankton, South Dakota, the 
beds are chalk}- throughout, the difference being doubtless due to 
the fact that the Sioux City area was nearer the shore line of the 
Cretaceous sea in which the beds were deposited. At Ponca Jn- 
oceramus is about as common as at Sioux Cit}', but the strata in 
which the shells ai'e embedded are lithologicallj' intermediate be- 
tween the condition of the /Moce?-amMs-bearing laj'ers at Sioux 
Citj- and the condition observed in the basal parts of the Niobrara 
group at St. Helena. Furthermore the beds referred b}' Haj^den 
to the Dakota and Fort Benton group are as well developed at Sioux 
