THE LANCE FORMATION 349 



closely to the Judith Eiver fauna; a brackish-water invertebrate fauna 

 developed locally in. its lower part, which is in part identical with the 

 Laramie fauna and is evidently derived from older Cretaceous faunas ; a 

 fresh-water invertebrate fauna which is in part restricted to the Lance, 

 in part identical with the Laramie, with a few forms passing up into the 

 Fort Union, but altogetlier much more closely allied to Cretaceous than 

 to Tertiary faunas, and a flora which is closely related to the Fort Union 

 flora and regarded by Knowlton as somewhat younger than the Denver 

 flora. 



DEVELOPMENT IN NORTH AND SOUTH DAKOTA 



Revieiv of investigations. — In 1910 I published evidence^ tending to 

 prove that in eastern Wyoming and the Dakotas there is gradual transi- 

 tion with practically continuous sedimentation from the Fox Hills sand- 

 stone into the Lance formation, and that the local erosion, of which there 

 is evidence, in some places could not represent any important time inter- 

 val. Stratigraphic details and faunal lists from localities in the Standing 

 Eock and Cheyenne River Indian reservations made it clear that the ero- 

 sion and other phenomena, believed to indicate a break, occurred while 

 the marine Cretaceous Sea was still present. In the same paper attention 

 was called to an oyster-bed in the Lance formation near Yule, on the 

 Little Missouri, North Dakota, in strata approximately 500 feet above the 

 base of the formation and a less distance above beds in the same neighbor- 

 hood containing Triceratops and other dinosaurs. This oyster-bed was 

 cited as evidence that the Cretaceous Sea was still near enough to send its 

 tidal brackish waters to the Yule locality. It is now known that that sea 

 lay to the eastward in southern ^NTorth Dakota and northern South Dakota, 

 and that its marine sediments, with a thickness of 200 or 300 feet, form- 

 ing a marine member of the Lance formation, cover the 400 feet of con- 

 tinental deposits belonging to the same formation which overlie the Fox 

 Hills sandstone in the Indian reservations just mentioned. It is not pos- 

 sible and it would not be appropriate for me to give the full evidence for 

 this statement at this time; but the facts of stratigraphy and areal dis- 

 tribution on which it is based are well known, and will be published in 

 detail in a series of reports in press and in preparation by members of 

 the Western Fuel section of the United States Geological Survey. These 

 reports describe a connected area extending from the Missouri River 

 across the Standing Rock and Cheyenne River reservations to the western 

 boundary of North and South Dakota. The Indian reservations are de- 



"> Fox Hills sandstone and Lance formation ("Ceratops Beds") in South Dakota and 

 eastern Wyoming. Amer. Jour. Sci., vol. 30, WIO, pp. 172-188. 

 XXV— Bull. Geol. Soc. Am., Vol. 25, 1913 



