T. W. Stanton — Fox Hills Sandstone. 187 



The list of fossils from No. 6 is as follows : 



Avicula nebrascana E. & S. 

 Avicula fibrosa M. & H. 

 Leda ? sp. 



Nucula 2'>lanimarginata M. & II. 

 Tellina scitula M. & H. 

 Lamna f sp. 



Tlie occiTi-rence of these fossils in JSTo. 6 and of Halxjmen- 

 ites at higher horizons shows that the strata to the top of the 

 white sandstones are mainly marine and belong to the Fox 

 Hills. In previous descriptions, only No. 10 was referred to 

 the Fox Hills and the age of the overlying 400 feet of sand- 

 stones was left doubtful. 



In the Lance Creek area the evidence for a gradual transi- 

 tion from the marine Fox Hills into the Lance formation is 

 found in the occurrence of a brackish-water bed at the top of 

 the marine strata, in the presence of coal beds and other 

 evidence of alternating land and marine conditions before the 

 close of the Fox Hills, and in the absence of any indication of 

 an impoi'tant unconformity anywhere in the section. 



As compared with sections in the other areas here discussed 

 the most striking feature of the Converse County sections is 

 the much greater thickness of the Fox Hills sandstone and of 

 the Lance formation. The average thickness of the Fox Hills 

 sandstone in the South Dakota sections described is little more 

 than 100 feet and it is still less at Marmarth, North Dakota, 

 while in Converse County it is 400 to 500 feet, making the 

 ratio about 1 to 4. The Lance formation shows about the same 

 ratio of increase and the marine Cretaceous formations beneath 

 the Fox Hills are also known to be much thicker in Converse 

 County. Farther south in Colorado these marine formations, 

 including the Fox Hills, show a still greater increase in thick- 

 ness. It is therefore reasonable to attribute the variation in 

 the thickness of the Fox Hills to variation in the rate of sedi- 

 mentation due to varying distances from the source of supply 

 and perhaps to variation in the height of the neighboi'ing 

 lands. That the varying thickness is not due to erosion from 

 the top is proved in the case of some of the thinnest sections 

 by the presence of the brackish-water beds at the top showing 

 the final stage in marine sedimentation. 



The three areas discussed in this paper taken together tell a 

 story of gradually changing conditions near the end of the 

 Cretaceous when the uplift of the Kocky Mountain region was 

 draining the interior sea. The uplift was not uniform nor 

 continuous and the emergence above sea level could not have 

 been simultaneous for all localities throughout the region. As 



