90 CONTRIBUTIONS TO PAL^ONTOLOGY 



there is little lithologic distinction between beds that contain characteristic Lance species and 

 others that contain characteristic Fox Hills species, and in these places the fossils must be made 

 the chief rcHancc for separating the formations." ' 



In keeping with these suggestions, the foUowing criteria for the practical de- 

 limitation of the Fox Hills sandstone in northeastern Colorado were subsequently 

 pubUshed by a committee of the Rocky Mountain Association of Petroleum Geolo- 

 gists after a field conference with Dr. Reeside: 



"The base of the Fox Hills formation shall be considered as the horizon below which the 

 section is prcdominantly gray marine clay shales and sandy shales of Pierre age, and above which 

 the section changes rapidly to a bufl to brown sandstone containing numerous large gray to brown, 

 hard sandy concretions. This lower concretionary member is commonly overlain by a series of 

 Hght gray to brown sandstones and sandy shales. 



"The top of the Fox Hills formation shall be considered as the horizon above which the sec- 

 tion is composed predominantly of fresh and brackish-water deposits accompanied bj' coals and 

 hgnitic shales, and below which it is predominantly marine." ^ 



Attempts by our field party to apply these practical criteria to the section in the 

 Lance Creek area showed that the Fox Hills sandstone, though somewhat variable 

 in thickness, averages about 250 to 350 feet in thickness, as has also been shown by 

 Dobbin and Reeside.^ The greater thicknesses reported by earUer writers are ap- 

 parently the result of the inclusion in the Fox HiUs of several massive, gray sand- 

 stones (Colgate equivalent?) which in places Ue above the tj^pical, fossiUferous Fox 

 HiUs beds. These massive sandstones are locally present in the Lance Creek area, 

 where they have the following characteristics : (1) they are lenticular bodies of sand, 

 which were traced laterally along the strike from just east of Localit}' P3652 into the 

 typical duU-gray sandy shales, lignites, and plant-bearing gray sandstones of the 

 lower Lance formation at LocaUty P3853; (2) they contain a well-defined lignite bed 

 above thc lowest massive sandstone, as well as several lignite lenses higher up; (3) 

 they contain no marine invertebrates ; (4) locally they have jielded both dinosaur 

 remains and impressions of land plants; and (5) they resemble more closely the 

 lenticular sandstones of the overlying Lance succession than any beds of the true 

 Fox Hills. On the basis of the criteria set forth above, these massive sandstones 

 are not considerod a part of the Fox Hills formation. 



The following section, measured by Stauton ■* at the mouth of Lance Creek, is 

 typical of the Fox HUIs sandstonc in this region, and shows the relation of the mas- 

 sive sandstones, which were regarded by Stanton as upper Fox Hills, to the beds 

 below. 



Seclion of Fox Hills Sandslone at Mouth of Lance Creek, Sec. 11, T. 39 N., R. 62 W., Wyoming 



Lance formation [part] : '^'infed^ 



White cros.s-bcdded sandstone with irrcgular brown indurated bands, masses, and 



concretions 50.0 



Soft sandy shale with bands of lignitic shale. Fragmeuts of dinosaur bone were 



found on the surfacc hcre 50. 



[Both units above were included by Stanton in tlie Fox Hills[ 



' Dobbin, C. E., and Reeside, J. B., Jr., op. cit., 11, 1929. 



' Lovering, T. S., Aurand, H. A., Lavington, C. S., and Wilson, J. H., Bull. Amer. Assoc. Petroleum Geologista, vol. 

 16, 702, 1932. 



• Dobbin, C. E., and Reesidc, J. B., Jr., op. cit., 19, 1929. 



* Stanton. T. W., Amer. Jour. Sci., 4th ser., vol. 30, 184, 1910. 



