256 T. W. STANTON DAKOTA SANDSTONE PROBLEMS 



the first collections of both plants and mollusks were obtained are dis- 

 tributed along both banks of the Missouri from Ponca, Nebraska, to 

 Tekamah, Nebraska, a distance of about 60 miles. The total thickness 

 of exposed rocks referable to the Dakota in the neighborhood of Sioux 

 City, from the mouth of Big Sioux River to Sargents Bluff, is about 100 

 feet, and if the section be extended down to Tekamah, so as to include 

 the plant-bearing beds at that place, at Decatur, and at Blackbird Hill, 

 it will still probably cover less than the upper 200 feet of the Dakota 

 sandstone. The invertebrates described from the mouth of Big Sioux 

 River must have come from the upper 50 feet of the formation and prob- 

 ably from the uppermost sandstone bed. The following are figured by 

 Meek : 2 



Trigonarca siou.rcnsis (Hall and Meek). 



Margaritana nebrasccnsis Meek. 



ArcopageUaf macrodonta Meek. 



("arena dakotcnsis Meek and Hayden. 



/'ha rclla dakotcnsis Meek and Hayden. 



Mwctra sioujcensis Meek and Hayden. 



Cast of the burrow of some unknown mollusk. 



Another fossiliferous locality in the same neighborhood, discovered by 

 C. N. Gould 3 near Jackson, Nebraska, and collected from by myself in 

 1920, is in the upper 10 feet of the Dakota and has yielded the following 

 forms : 



Ostrea sp. 



Trigonarca siouxensis (Hall and Meek). 



Arcopagella? macrodonta Meek? 



PhareUaf sp. 



Corbula hicksii White. 



Martesia? sp. casts of burrows in wood. 



Pscudomelaniaf sp. 



Yivipara hicksi White? 



Volutodermat sp. 



Vertebra of fish. 



Vertebra of turtle. 



Tooth of crocodile. 



Most of the invertebrates of these two lists are marine, but an admix- 

 ture of fresh and brackish water forms is indicated by the species referred 

 to Margaritana, Pharella, Cyrena, and Vivipara. The sediments also 

 indicate a struggle between sea and land along a low sandy shore with 

 swamps, lagoons, and inlets. There are rapid changes from sand to shale, 



3 U. S. Geol. Survey of the Territories, vol. 9, pi. i. 1870. 

 3 Kansas Acad. Sci. Trans., vol. 17, 1901, p. 108. 



