270 T. \V. STANTON DAKOTA SANDSTONE PROBLEMS 



zones are recognizable., including the Niobrara, the zone of Prionocyclus 

 Wyoming ensis, the zone of Inoceramus labiatiis, and a basal zone whose 

 fauna connects it Avith the basal part of the Mancos in southern Utah 

 and to a slight extent with the fauna of the Woodbine sand of Texas. 



Review and Conclusion 



The map (plate 5) shows the location of the sections discussed and 

 indicates the areal distribution of the Kiowa, or Gryphcea corrugata, 

 fauna, of the Mentor, or more restricted Washita fauna, and the more or 

 less hypothetical extent of the sea in which the oldest marine Cretaceous 

 deposits of northern Colorado and of Wyoming and more northern re- 

 gions were laid down. 



The paleogeography and history of the Great Plains in Washita and 

 Dakota time can not be correctly interpreted until it is possible to make 

 positive correlations between the well determined Washita of the southern 

 area and the doubtful beds farther north. Grabau 19 has interpreted the 

 Dakota as deposited during a retreat and readvance of the sea, the retreat 

 beginning early in Washita time and the readvance ending in Benton 

 time. It seems to me impossible to adjust the known facts to this hy- 

 pothesis. The encroachment of the sea on the American continent which 

 inaugurated Trinity (earliest Comanche) time was gradually continued 

 through Trinity and Fredericksburg time, but at the beginning of the 

 Washita the movement was accelerated without any previous reversal and 

 the sea soon reached Kansas and Colorado. I agree with Twenhofel's 20 

 suggestion that it is reasonable to regard the Cheyenne, Kiowa, Mentor, 

 and associated rocks as all deposited during one general transgression of 

 the sea, but I would interpret the Dakota as representing merely a halt 

 in that advance rather than a complete retreat of the sea from the Great 

 Plains area as far south as Texas. The axis of depression in which this 

 irregular advance was made was probably near the western border of the 

 Plains, and from this trough the waters spread laterally as the depression 

 increased. Littoral, lagoon, and swamp deposits were formed along both 

 shores of the comparatively narrow, shallow sea, and locally the two kinds 

 of deposits shifted places from time to time. In my opinion, the sea. did 

 not go beyond southern Colorado and central Kansas during Washita 

 time. 



If the views here stated are true, it is obvious that there is no proper 

 boundary between two geologic systems either at the base or at the top of 



18 Bull. Geol. Soc. Am., vol. 17. 1000. pp. 020-627. Principles of stratigraphy, p. 739. 

 20 Am. Jour. Sci., 4th ser., vol. 40, 1020, pp. 280-204. 



