T. W. Stanton — Fox Hills Sandstone. 181 



adopts the curroiit interpretation of the evidence from fossil 

 plants which refers the Lance formation to the Fort Union 

 formation, assigned to the Eocene, and reqnires a period 

 between tlie close of the Fox Hills and the heginnino; of the 

 Lance formation long enough to represent the Laramie, Ara- 

 pahoe, and Denver formations of the Denver Basin, together 

 with the long intervals of erosion which preceded the Arapa- 

 hoe and the Denver, and another interval supposed to be 

 between tlie Denver and tlie Fort Union. It has been asserted 

 that the erosion preceding Arapahoe deposition cut down 

 through 14,000 feet of stratified rocks in the Denver area. It 

 is worthy of note that there are no Fort Union rocks or floras 

 known in the Denver Basin and no plant-bearing formations 

 older than the Lance formation in the ai"ea now under dis- 

 cussion. 



Early in their field work last summer the geologists had 

 determined that the Lance formation immediately overlies the 

 Fox Hills, and had received reports that the plants from near 

 their base belong to the Fort Union flora. Special search was 

 therefore made for physical evidence of the unconformity and 

 hiatus which the facts seemed to demand. The only horizon 

 at which a somewhat generally distributed break was suggested 

 is at the iri-egular base of the brackish-water bed, and this 

 irregular surface was therefore tentatively taken as the base of 

 the Lance formation on the supposition that it indicated a 

 hiatus in the sedimentary record equivalent to the Laramie 

 and post- Laramie formations of the Denver Basin. That there 

 was no such break in the record at this horizon is definitely 

 proved by the occurrence of ammonoids and other fossils char- 

 acteristic of the marine Fox Hills in the brackish-water bed 

 as already described. The brackish-water fossils in themselves 

 give evidence of their Cretaceous age. In the first place, 

 almost all of them belong to species that in other areas occur in 

 unqnestioned Cretaceous formations. Secondly, brackish waters 

 with such a fauna as the one in question must have had con- 

 nection with the sea and no connection with Tertiary seas is 

 probable. Marine waters retreated from the region of the 

 Rocky Mountains and Great Plains at or just before the close 

 of the Cretaceous, and there is no evidence that the sea has 

 approached the region since that time. The nearest marine 

 deposits of Eocene or later age are in the lower Mississippi 

 valley and in the Gulf coastal plain of Texas. This fact gives 

 special importance to the occurrence of an oyster bed 500 feet 

 above the base of the Lance formation on the Little Missouri 

 River, !North Dakota, which will be described on a subsequent 

 page. 



It may be suggested that there is a stratigraphic break 

 between the brackish-water and fresh-water horizons, but no 



