6 C O N T R I B U T I O N S TO PAL^ONTOLOGY 



lenticularis, were shown to come from strata included within the basal part of the 

 Medicine Bow formation and not from the sandstones within the upper part of the 

 Lewis shale. The wTiter's field studies and coUections, which are later described in 

 detail, are in agreement with these observations. It is clearly evident, therefore, 

 that sediments of Fox Hills age form a part of what has hitherto been regarded as 

 the basal portion of the Medicine Bow formation. The recognition of the Fox Hills 

 formation in the region must then depend on whether or not it can be distinguished 

 as a separable lithologic unit. The foUowing criteria for the recognition and de- 

 Hmitation of this formation in northeastern Colorado have recently been pubUshed 

 by a committee of the Rocky Mountain Association of Petroleum Geologists ^ 

 after a field conference with Dr. J. B. Reeside, Jr.: 



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

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

 the section changes rapidly to a bu£f to brown sandstone containing numerous large gray to brown, 

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

 light gray to brown sandstones and sandy shales. 



"The top of the Fox Hiils formation shall be considered as the horizon above which the 

 section is composed predominantly of fresh and brackish-water deposits accompanied by coals 

 and lignitic shales, and below which it is predominantly marine." 



The appUcation of these criteria to the basal 400 ± feet of the Medicine Bow 

 formation has recently been made by Conway in an unpubUshed report.^ He has 

 pointed out that aU of the above requirements are exactlj^ fulfiUed in the sections 

 which he studied near Corson Ranch, near Walcott, and near RawUns in this area. 

 It is interesting to note that the lowest coal beds in the sections are invariably at 

 the horizon which separates the underlying marine faunas from the overlying fresh 

 and brackish-water faunas, and which also separates the massive, more persistent 

 sandstones below from the tliinner, lenticular sandstones above. For practical 

 purposes of mapping, therefore, the lowest coal beds can be regarded as the base 

 of the Medicine Bow formation. 



The horizons which have yielded plant remains aU Ue within the basal 1300 feet 

 of the Medicine Bow formation, as here redefined. Each horizon has been ac- 

 curately located with reference to the characteristic Fox HiUs fauna, of which the 

 ammonite Sphenodiscus lenticularis is regarded as the most trustworthy and 

 diagnostic index species. It is certain, therefore, that the evidence from strati- 

 graphic position and from marine invertebrates indicates post-Fox HUls age for 

 the plant-bearing horizons. Their pre-Fort Union age is equaUy clearly indicated 

 by the presence of ceratopsian dinosaurs and invertebrates of Lance age in the 

 Medicine Bow formation, and Triceratops dinosaurs in the basal beds of the over- 

 lying Ferris formation.^ The combined geologic data, exclusive of the plant evi- 

 dence, are, therefore, consistent throughout in supporting a reference of the Medicine 

 Bow formation as of Lance age, or more specificaUy as of HeU Creek age. The 



" Lovering, T. S., Aurand, H. A., Lavington, C. S., and WiUon, J. H., Fox Hills Formation, Northeastern Colorado. 

 Amer. Aesoc. Petroleum Geologists BuU., vol. 16, 702. 1932. 



« Conway, W. P.. Jr., Slraiigraphy and Paleonlology of the Medicine Bow Formation, Hanna Baain, Wyoming, Dept. 

 Geology, Princeton University, senior thesis, 30-32, 1936. 



2 Dobbin, C. E., Bowen, C. F., and Hoota, H. W., U. S. Geol. Survey BuU. 804, 25, 1929. 



