IOWA ACADEMY OF SCIENCES. 9 



that it is difficult to determine their stratigraphical equivalents without actual 

 comparison, which it has thus far been impossible to make. There is no doubt, 

 however, that all the Iowa Cretaceous strata belong to the " Earlier Cretaceous" 

 of Meek and Hayden, nor any doubt that the lower portion of ours is equivalent 

 to a part of their Dakota group." 



Without attempting, therefore, to synchronize the Cretaceous of Iowa with the 

 Cretaceous formations studied by Meek and Hayden, Dr. White applies to all the 

 strata at Sioux City lying below the chalk, the name of The Woudbun/ Sandstones 

 and Shales, while the calcareous deposits, composed of chalk and soft Inoceramus- 

 bearing limestone, he calls the Ivoceranius Beds. 



Below the mouth of Iowa Creek, about three miles east of Ponca, Nebraska, the 

 Missouri river washes the foot of a high blulf in which Cretaceous strata, identical 

 in a,ll essential respects with those seen in Iowa above the mouth of the Big Sioux, 

 are exposed to a height of more than a hundred feet. The several beds of the pre- 

 ceding section, from 2 to 8 inclusive, are easily recognized, and at the summit of 

 the section, cropping out from beneath the thick mantle of loess, are indications of 

 the chalky beds of number 9. Farther up the river, almost directly north of Ponca, 

 there is another splendid natural section which is more than a mile in length and 

 at least 1.50 feet in height. At the base of the section are the beds seen below the 

 iwouth of Iowa Creek, while above all the sandstones and shales lie from twenty- 

 tive to thirty feet of rather hard chalk and Inoceramus-bearing limestone. There 

 can be no doubt that the beds near Ponca, Nebraska, are the exact equivalents of 

 beds in Iowa. Indeed, one may look away from the exposure at the bend east of 

 Ponca, across the plain which is here the combined valley of the Missouri and Big 

 Sioux, for a distance of only ten or twelve miles to the corresponding exposures in 

 Iowa. In the two blutfs that look toward each other from opposite sides of the 

 plain, you may trace the same succession of strata that, but tor the erosion of the 

 two great streams, would still be continuous across the intervening space. Further- 

 more, the beds are about equally well developed on both sides of the valley. 



Now, the exposure at the bend of the Missouri three miles below Ponca, Nebraska, 

 is described in detail by Hayden.* The chalky, marly or calcareous beds, which 

 are the exact equivalent of the Inoceramus beds of Iowa, are referred to the Nio- 

 brara group. The dark colored shale, identical with number 8 of the preceding sec- 

 tion, is called the Fort Benton group, while all the complex mass of alternating 

 sandstones and shales in the basal portion of the exposure is recognized as belong- 

 ing to the Dakota group. 



Between Ponca and St. James, about thirty miles on a direct line further up the 

 Missouri, the chalky beds of the Niobrara group crop out on all the higher hill 

 tops. The village of St. James is situated in the valley of Bow Creek, below the 

 level of the chalk. In the eastern edge of the village is an exposure of Fort Ben- 

 ton shales presenting the same characteristics as seen at a recent land slide on the 

 farm of Williams and Smith, a few miles north of Sioux City, in Iowa, and at the 

 exposure near Ponca, Nebraska. This shale furnishes a very perfect skeleton of a 

 saurian, as it was penetrated in digging a cistern on Sect. 8.5, T. 90, R 47., on 

 the Iowa side of the Big Sioux. Another similar skeleton that was carried about 

 the country some years ago lor exhibition purposes, was taken from the same hor- 

 izon near Ponca. A lew weeks before my visit a portion of a skeleton, forty feet 

 in length, was uncovered in excavating in Fort Benton shales near St, James. On 



♦First Annual Report of the United States Geological Survey of the Territories. 

 1867, pp. 47 and 4S. 



