10 IOWA ACADEMY OF SCIENCES. 



the tops or the hills near Bow ('reek, the dark Fort Benton shales are succeeded by 

 white or cream-colored chalk of the Niobrara division. 



St. Helena, about eight or nine miles above St. James, is situated on a higher 

 bluff, one hundred and thirty or one hundred and forty feet above the level of the 

 Missouri river. The bluff rises as a vertical wall almost from the edge of the 

 water. Between the river and the vertical escHrpment, the base of the bluff is con- 

 cealed by a talus composed chiefly of great blocks of chalk, but above the talus, 

 and rising to a height of forty feet above the level of the water, is an excellent ex- 

 posure of the dark shales of the Fort Benton group, differing in no essential re- 

 spect from the corresponding shales exposed at the land slide above the creamery 

 of Williams and Smith, or the shales occupying the same stratigraphical position 

 near Ponca and St. James. Above the Fort Benton shales lie ninety feet of soft 

 chalk belonging to the ISiobrara. The Niobrara beds at St. Helena exhibit some 

 points of difference from those seen on the Big Sioux, or on the Missouri across the 

 valley in Nebraska. The valves of Inoceramus are no longer present in such num- 

 bers, but some of the layers are crowded with Ostrea congesta. One impression of 

 the peculiarly corrugated muscular scar of Haploscaphn grandis was noticed. The 

 beds are uniformly chalky throughout, no part of the deposit being as much in- 

 durated as the Jnoceramus-bearing beds nenr Ponca and Sioux City. The expos- 

 ure at St. Helena is probably one of the most striking and interesting along the 

 river, and Hayden refers to it time and again in the work already cited. 



At Yankton, South Dakota, a short distance above St. Helena, and on the oppo- 

 site side of the Missouri, the Niobrara beds are developed in great force. A large 

 factory has been established about three miles west of Yankton to utilize the chalk 

 in the manufacture of Portland cement. The part of the formation at present 

 worked into cement lies above that exposed in the bluffs at St. Helena. It pre- 

 sents 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 disappeared beneath the level of the river; at all events they lie below the 

 level of any observed exposures. On the hilltops above the cement factory the 

 chalk of the Niobrara is overlain by the shales of the Fort Pierre group. Hayden 

 speaks of this group as making its appearance on the summit of the hills, near the 

 mouth of the Niobrara, but he might have found it thirty miles farther east de- 

 veloped 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 by no 

 means uncommon in the shaly portions of the Dakota group near Ponca and Sioux 

 City. 



With reference to the taxonomy of our Iowa .section, beds 1 to 7 inclusive are the 

 stratigraphical equivalents of beds near Ponca, Nebraska, which Hayden refers to 

 the Dakota group. No. 8 includes beds that at Ponca and St. Helena have been 

 referred to the Fort Benton group by the same author, and the Ivocerannis beds 

 No. 9, are the exact equivalents of the lower twenty cr thirty feet of the Niobrara 

 group. A part of the Inoceramus beds near Sioux City 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 chalky 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, Inoceramus is about as common as at Sioux City, but 

 the strata in which the cells are embedded are lithologically intermediate between 



